Immunology
Class 12 | Pure Science — Zoology | Chapter 8
Class 12 Unit 8 Pure_Science
Unit 3 | Chapter 8
Immunology
"Natural forces within us are the true healers of disease." — Hippocrates
Chapter Outline
8.1 Basic concepts of immunology | 8.2 Innate immunity | 8.3 Acquired immunity | 8.4 Immune responses | 8.5 Lymphoid organs | 8.6 Antigens | 8.7 Antibodies | 8.8 Antigen-antibody interactions | 8.9 Vaccines | 8.10 Vaccination and immunization | 8.11 Hypersensitivity | 8.12 Immunodeficiency diseases | 8.13 Autoimmune diseases | 8.14 Tumour immunology
Learning Objectives
- Understands the basic concepts of immunology.
- Differentiates between innate immunity and acquired immunity, primary immune response and secondary immune response, active and passive immunity.
- Realizes the importance of immunization.
- Learns to comprehend the concept of hypersensitivity.
In the previous chapter, we have studied in detail the various infections which cause diseases in human beings. In this chapter, we shall discuss how our body protects us from these infections by the effective mechanism of the immune system.
8.1 Basic Concepts of Immunology
Immunology is the study of immune system. This system protects an individual from various infective agents. It refers to all the mechanisms used by the body for protection from environmental agents that are foreign to the body. When the immune system does not function efficiently in an individual, it leads to infection causing disease.
The overall ability of body to fight against the disease causing pathogen is called immunity. It is also called disease resistance and the lack of immunity is known as susceptibility. Immunity is highly specific.
Normally many of the responses of the immune system initiate the destruction and elimination of invading organisms and any toxic molecules produced by them. These immune reactions are destructive in nature and are made in response only to molecules that are foreign to the host and not to those of host itself. This ability to distinguish foreign molecules from self is another fundamental feature of the immune system.
However, occasionally, it fails to make its distinction and reacts destructively against the host's own molecules; such autoimmune diseases can be fatal to the organism.
Almost all the macromolecules e.g. proteins, polysaccharides, nucleic acids, etc., as long as they are foreign to recipient organism can induce immune response. Any substance capable of eliciting immune response is called an ANTIGEN (ANTIbody GENerator).
There are two broad classes of immunity responses namely, innate immunity and acquired immunity.
8.2 Innate Immunity
Innate immunity is the natural phenomenon of resistance to infection which an individual possesses right from the birth. The innate defense mechanisms are non-specific in the sense that they are effective against a wide range of potentially infectious agents. It is otherwise known as non-specific immunity or natural immunity.
A number of innate defense mechanisms are operative non-specifically against a large number of microorganisms as shown in the Table 8.1.
Table 8.1: Innate Immunity — Types and Mechanisms
| Type of Innate Immunity | Mechanism |
1. Anatomical Barriers
Skin | Prevents the entry of microbes. Its acidic environment (pH 3-5) retards the growth of microbes. |
| Mucus membrane | Mucus entraps foreign microorganisms and competes with microbes for attachment. |
2. Physiological Barriers
Temperature | Normal body temperature inhibits the growth of pathogens. Fever also inhibits the growth of pathogens. |
| Low pH | Acidity of gastric secretions (HCl) kills most ingested microbes. |
| Chemical mediators | Lysozyme acts as antibacterial agent and cleaves the bacterial cell wall. Interferons induce antiviral state in the uninfected cells. Complementary substances produced from leucocytes lyse the pathogenic microbes or facilitate phagocytosis. |
| 3. Phagocytic Barriers | Specialized cells (Monocytes, neutrophils, tissue macrophages) phagocytose, and digest whole microorganisms. |
| 4. Inflammatory Barriers | Tissue damage and infection induce leakage of vascular fluid, containing chemotactic signals like serotonin, histamine and prostaglandins. They influx the phagocytic cells into the affected area. This phenomenon is called diapedesis. |
8.3 Acquired Immunity
The immunity that an individual acquires after birth is known as acquired immunity. It is the body's resistance to a specific pathogen. The unique features of acquired immunity are antigenic specificity, diversity, recognition of self and non-self and immunological memory.
Components of Acquired Immunity
Acquired immunity has two components: cell mediated immunity (CMI) and antibody mediated immunity or humoral immunity.
1. Cell Mediated Immunity: When pathogens are destroyed by cells without producing antibodies, then it is known as cell mediated immune response or cell mediated immunity. This is brought about by T cells, macrophages and natural killer cells.
2. Antibody Mediated Immunity or Humoral Immunity: When pathogens are destroyed by the production of antibodies, then it is known as antibody mediated or humoral immunity. This is brought about by B cells with the help of antigen presenting cells and T helper cells. Antibody production is the characteristic feature of vertebrates only.
Types of Acquired Immunity
Acquired immunity may be active immunity or passive immunity.
Table 8.2: Differences between Active and Passive Immunity
| S.No. | Active Immunity | Passive Immunity |
| 1 | Active immunity is produced actively by host's immune system. | Passive immunity is received passively and there is no active host participation. |
| 2 | It is produced due to contact with pathogen or by its antigen. | It is produced due to antibodies obtained from outside. |
| 3 | It is durable and effective in protection. | It is transient and less effective. |
| 4 | Immunological memory is present. | No memory. |
| 5 | Booster effect on subsequent dose is possible. | Subsequent dose is less effective. |
| 6 | Immunity is effective only after a short period. | Immunity develops immediately. |
The immunological resistance developed by the organisms through the production of antibodies in their body is called active immunity. Active immunity is acquired through the use of a person's immune responses, which lead to the development of memory cells. Active immunity results from an infection or an immunization.
Passive immunity does not require the body to produce antibodies to antigens. The antibodies are introduced from outside into the organism. Thus, passive immunity is acquired without the activation of a person's immune response, and therefore there is no memory.
Do You Know?
The process of production of blood cells in the bone marrow is called haematopoiesis.
8.4 Immune Responses
The immune responses may be primary or secondary.
Table 8.3: Differences between Primary and Secondary Immune Responses
| S.No. | Primary Immune Response | Secondary Immune Response |
| 1 | It occurs as a result of primary contact with an antigen. | It occurs as a result of second and subsequent contacts with the same antigen. |
| 2 | Antibody level reaches peak in 7 to 10 days. | Antibody level reaches peak in 3 to 5 days. |
| 3 | Prolonged period is required to establish immunity. | It establishes immunity in a short time. |
| 4 | There is rapid decline in antibody level. | Antibody level remains high for longer period. |
| 5 | It appears mainly in the lymph nodes and spleen. | It appears mainly in the bone marrow, followed by the spleen and lymph nodes. |
Primary Immune Response
The primary immune response occurs when a pathogen comes in contact with the immune system for the first time. During this, the immune system has to learn to recognize the antigen, produce antibody against it and eventually produce memory lymphocytes. The primary immune response is slow and short-lived.
Secondary Immune Response
The secondary immune response occurs when a person is exposed to the same antigen again. During this time, immunological memory has been established and the immune system can start producing antibodies immediately. Within hours after recognition of the antigen, a new army of plasma cells are generated. Within 2 to 3 days, the antibody concentration in the blood rises steeply to reach much higher level than primary response. This is also called as "booster response".
8.5 Lymphoid Organs
Immune system of an organism consists of several structurally and functionally different organs and tissues that are widely dispersed in the body. The organs involved in the origin, maturation and proliferation of lymphocytes are called lymphoid organs.
Based on their functions, they are classified into primary or central lymphoid organs and secondary or peripheral lymphoid organs. The primary lymphoid organs provide appropriate environment for lymphocytic maturation. The secondary lymphoid organs trap antigens and make it available for mature lymphocytes, which can effectively fight against these antigens.
Primary Lymphoid Organs
Bursa of Fabricius of birds, bone marrow and thymus gland of mammals constitute the primary lymphoid organs involved in the production and early selection of lymphocytes. These lymphocytes become dedicated to a particular antigenic specificity. Only when the lymphocytes mature in the primary lymphoidal organs, they become immunocompetent cells. In mammals, B cell maturation occurs in the bone marrow and T cells maturation occurs in the thymus.
Thymus
The thymus is a flat and bilobed organ located behind the sternum, above the heart. Each lobe of the thymus contains numerous lobules, separated from each other by connective tissue called septa. Each lobule is differentiated into two compartments — the outer compartment or outer cortex, is densely packed with immature T cells called thymocytes, whereas the inner compartment or medulla is sparsely populated with mature thymocytes. One of its main secretions is the hormone thymosin. It stimulates the T cell to become mature and immunocompetent. By the early teens, the thymus begins to atrophy and is replaced by adipose tissue. Thus, thymus is most active during the neonatal and pre-adolescent periods.
Bone Marrow
Bone marrow is a lymphoid tissue found within the spongy portion of the bone. Bone marrow contains stem cells known as haematopoietic cells. These cells have the potential to multiply through cell division and either remain as stem cells or differentiate and mature into different kinds of blood cells.
Secondary or Peripheral Lymphoid Organs
In secondary or peripheral lymphoid organs, antigen is localized so that it can be effectively exposed to mature lymphocytes. The best examples are lymph nodes, appendix, Peyer's patches of gastrointestinal tract, tonsils, adenoids, spleen, MALT (Mucosal-Associated Lymphoid Tissue), GALT (Gut Associated Lymphoid Tissue), BALT (Bronchial/Tracheal-Associated Lymphoid Tissue).
Peyer's Patches
Peyer's patches are oval-shaped areas of thickened tissue that are embedded in the mucus-secreting lining of the small intestine of humans and other vertebrate animals. Peyer's patches contain a variety of immune cells, including macrophages, dendritic cells, T cells, and B cells.
Tonsils
The tonsils (palatine tonsils) are a pair of soft tissue masses located at the back of the throat (pharynx). The tonsils are part of the lymphatic system, which help to fight infections. They stop invading germs including bacteria and viruses.
Spleen
Spleen is a secondary lymphoid organ located in the upper part of the abdominal cavity close to the diaphragm. Spleen contains B and T cells. It brings humoral and cell mediated immunity.
Do You Know?
The adenoids are glands located in the roof of the mouth, behind the soft palate where the nose connects to the throat. The adenoids produce antibodies that help to fight infections. Typically, the adenoids shrink during adolescence and may disappear by adulthood.
Lymph Node
Lymph node is a small bean-shaped structure and is part of the body's immune system. It is the first one to encounter the antigen that enters the tissue spaces. Lymph nodes filter and trap substances that travel through the lymphatic fluid. They are packed tightly with white blood cells, namely lymphocytes and macrophages. There are hundreds of lymph nodes found throughout the body. They are connected to one another by lymph vessels.
Lymph is a clear, transparent, colourless, mobile and extracellular fluid connective tissue. As the lymph percolates through the lymph node, the particulate antigen brought in by the lymph will be trapped by the phagocytic cells, follicular and interdigitating dendritic cells.
Lymph node has three zones: the cortex, paracortex and medulla. The outer most layer of the lymph node is called cortex, which consists of B-lymphocytes, macrophages, and follicular dendritic cells. The paracortex zone is beneath the cortex, which is richly populated by T lymphocytes and interdigitating dendritic cells. The inner most zone is called the medulla which is sparsely populated by lymphocytes, but many of them are plasma cells, which actively secrete antibody molecules.
As the lymph enters, it slowly percolates through the cortex, paracortex and medulla, giving sufficient chance for the phagocytic cells and dendritic cells to trap the antigen brought by the lymph. The lymph leaving a node carries enriched antibodies secreted by the medullary plasma cells against the antigens that enter the lymph node.
Sometimes visible swelling of lymph nodes occurs due to active immune response and increased concentration of lymphocytes. Thus swollen lymph nodes may signal an infection. There are several groups of lymph nodes. The most frequently enlarged lymph nodes are found in the neck, under the chin, in the armpits and in the groin.
MALT, GALT and BALT
The mucosa associated lymphoid tissue (MALT) is a diffuse system of small concentrations of lymphoid tissue in the alimentary, respiratory and urino-genital tracts. MALT is populated by lymphocytes such as T and B cells, as well as plasma cells and macrophages, each of which is well situated to encounter antigens passing through the mucosal epithelium. It also possesses Immunoglobulin A antibodies.
Gut-associated lymphoid tissue (GALT) is a component of the mucosa associated lymphoid tissue (MALT) which works in the immune system to protect the body from invasion in the gut.
Bronchus Associated Lymphoid Tissues (BALT) also a component of MALT is made of lymphoid tissue (tonsils, lymph nodes, lymph follicles) found in the respiratory mucosae from the nasal cavities to the lungs.
Cells of the Immune System
The immune system is composed of many interdependent cells that protect the body from microbial infections and the growth of tumour cells.
Table 8.4: The Cellular Composition of Adult Human Blood
| Cell Type | Number of Cells per µl | Approximate Percentage |
| Red blood cells | 42 lakhs to 65 lakhs | — |
| White blood cells — Agranulocytes |
| Lymphocytes | 1500 to 4000 | 20 to 30 |
| Monocytes | 200 to 950 | 2 to 7 |
| White blood cells — Granulocytes |
| Neutrophils | 2000 to 7000 | 50 to 70 |
| Basophils | 50 to 100 | Less than 1 |
| Eosinophils | 40 to 500 | 2 to 5 |
| Platelets | 1 lakh 50 thousand to 5 lakhs | — |
All these cells are derived from pluripotent haematopoietic stem cells. Each stem cell has the capacity to produce RBC, WBC and platelets. The only cells capable of specifically recognizing and producing an immune response are the lymphocytes. The other types of white blood cells play an important role in nonspecific immune response, antigen presentation and cytokine production.
Lymphocytes
About 20 to 30% of the white blood cells are lymphocytes. They have a large nucleus filling most of the cell, surrounded by a little cytoplasm. The two main types of lymphocytes are B and T lymphocytes. Both these are produced in the bone marrow.
B lymphocytes (B cells) stay in the bone marrow until they are mature. Then they circulate around the body. Some remain in the blood, while others accumulate in the lymph nodes and spleen.
T lymphocytes leave the bone marrow and mature in the thymus gland. Once mature, T cells also accumulate in the same areas of the body as B cells.
Lymphocytes have receptor proteins on their surface. When receptors on a B cell bind with an antigen, the B cell becomes activated and divides rapidly to produce plasma cells. The plasma cells produce antibodies. Some B cells do not produce antibodies but become memory cells. These cells are responsible for secondary immune response.
T lymphocytes do not produce antibodies. They recognize antigen presenting cells and destroy them. The two important types of T cells are Helper T cells and Killer T cells. Helper T cells release a chemical called cytokine which activates B cells. Killer cells move around the body and destroy cells which are damaged or infected.
Apart from these cells, neutrophils and monocytes destroy foreign cells by phagocytosis. Monocytes when they mature into large cells, they are called macrophages which perform phagocytosis on any foreign organism.
Dendritic Cells
Dendritic cells are called so because they are covered with long, thin membrane extensions that resemble dendrites of nerve cells. These cells present the antigen to T helper cells. Four types of dendritic cells are known. They are langerhans, interstitial cells, myeloid and lymphoid cells.
8.6 Antigens
The term antigen (Ag) is used in two senses, the first to describe a molecule which generates an immune response and the second, a molecule which reacts with antibodies. In general antigens are large, complex molecular substances that can induce a detectable immune response. Thus, an antigen is a substance that is specific to an antibody or a T cell receptor and is often used as a synonym for immunogen.
Do You Know?
The histocompatibility antigens are cell surface antigens that induce an immune response leading to rejection of allografts.
An immunogen is a substance capable of initiating an immune response.
Haptens are substances that are non-immunogenic but can react with the products of a specific immune response.
Substances that can enhance the immune response to an antigen are called adjuvants.
An epitope is an antigenic determinant and is the active part of an antigen.
A paratope is the antigen binding site and is a part of an antibody which recognizes and binds to an antigen.
Do You Know?
Antigenicity is the property of a substance (antigen) that allows it to react with the products of the specific immune response.
Types of Antigens
On the basis of origin, antigens are classified into exogenous antigens and endogenous antigens.
The antigens which enter the host from the outside in the form of microorganisms, pollens, drugs, or pollutants are called exogenous antigens.
The antigens which are formed within the individual are endogenous antigens. The best examples are blood group antigens.
8.7 Antibodies
Antibodies are immunoglobulin (Ig) protein molecules synthesized on exposure to antigen that can combine specifically with the antigen. Whenever pathogens enter our body, the B lymphocytes produce an army of proteins called antibodies to fight with them. Thus, they are secreted in response to an antigen (Ag) by the effect of B cells called plasma cells.
The antibodies are classified into five major categories, based on their physiological and biochemical properties. They are:
- Immunoglobulin G (gamma)
- Immunoglobulin M (mu)
- Immunoglobulin A (alpha)
- Immunoglobulin D (delta)
- Immunoglobulin E (epsilon)
In the 1950s, experiments by Porter and Edelman revealed the basic structure of the immunoglobulin. An antibody molecule is Y shaped structure that comprises of four polypeptide chains, two identical light chains (L) of molecular weight 25,000 Dalton (approximately 214 amino acids) and two identical heavy chains (H) of molecular weight 50,000 Dalton (approximately 450 amino acids). The polypeptide chains are linked together by di-sulphide (S-S) bonds. One light chain is attached to each heavy chain and two heavy chains are attached to each other to form a Y shaped structure. Hence, an antibody is represented by H₂L₂.
The heavy chains have a flexible hinge region at their approximate middles. Each chain (L and H) has two terminals — C terminal (Carboxyl) and amino or N terminal. Each chain (L and H) has two regions — a variable (V) region at one end and a much larger constant (C) region at the other end.
Antibodies responding to different antigens have very different (V) regions but their (C) regions are the same in all antibodies. In each arm of the monomer antibody, the (V) regions of the heavy and light chains combines to form an antigen binding site shaped to 'fit' a specific antigenic determinant. Consequently each antibody monomer has two such antigen binding regions.
The (C) regions that forms the stem of the antibody monomer determine the antibody class and serve common functions in all antibodies.
Functions of Immunoglobulin
The functions of immunoglobulin are agglutination, precipitation, opsonisation, neutralization, etc.
8.8 Antigen Antibody Interaction
The reaction between an antigen and antibody is the basis for humoral immunity or antibody mediated immunity. The reaction between antigen and antibody occurs in three stages:
- During the first stage, the reaction involves the formation of antigen antibody complex.
- The next stage leads to visible events like precipitation, agglutination, etc.
- The final stage includes destruction of antigen or its neutralization.
Binding Force of Antigen Antibody Reaction
The binding force between antigen and antibody is due to three factors:
- Closeness between antigen and antibody
- Non-covalent bonds or intermolecular forces (hydrogen bonds, electrostatic bonds, Van der Waals forces and hydrophobic bonds)
- Affinity of antibody — the strength of the reaction between a single antigenic determinant and a single combining site on the antibody
When antigen and antibody are closely fitted, the strength of binding is great. When they are apart, binding strength is low.
Applications of Antigen Antibody Reactions
- To determine blood groups for transfusion
- To study serological ascertainment of exposure to infectious agents
- To develop immunoassays for the quantification of various substances
- To detect the presence or absence of protein in serum
- To determine the characteristics of certain immunodeficiency diseases
Different Types of Antigen and Antibody Reactions
Precipitin Reaction: The reaction between soluble antigen and antibody leads to visible precipitate formation. Antibodies that bring about precipitate formation on reacting with antigens are called precipitins.
Agglutination Reaction: Whenever a particulate antigen interacts with its antibody, it would result in clumping or agglutination of the particulate antigen. The antibody involved in bringing about agglutination reaction is called agglutinin.
Opsonisation: Opsonisation or enhanced attachment is the process by which a pathogen is marked for ingestion and destruction by a phagocyte. Opsonisation involves the binding of an opsonin (i.e., antibody) to a receptor on the pathogen's cell membrane. After opsonin binds to the membrane, phagocytes are attracted to the pathogen. This results in a much more efficient phagocytosis.
Neutralization Reactions: The neutralization reactions are the reactions of antigen-antibody that involve the elimination of harmful effects of bacterial exotoxins or a virus by specific antibodies. These neutralizing antibodies are known as antitoxins. This specific antibody is produced by a host cell in response to a bacterial exotoxin or corresponding toxoid (inactivated toxin).
8.9 Vaccines
A vaccine is a biological preparation that provides active acquired immunity to a particular disease and resembles a disease causing microorganism and is often made from weakened or attenuated or killed forms of the microbes, their toxins, or one of its surface proteins.
Vaccines "teach" our body how to defend itself when viruses or bacteria invade it. Vaccines deliver only very little amounts of inactivated or weakened viruses or bacteria, or parts of them. This allows the immune system to recognize the organism without actually experiencing the disease. Some vaccines need to be given more than once (i.e., a 'booster' vaccination) to make sure the immune system can overcome a real infection in the future. Vaccine initiates the immunization process.
Classification of Vaccines
The vaccines are classified as first, second and third generation vaccines.
First Generation Vaccines are further subdivided into:
- Live attenuated vaccines — use the weakened (attenuated), aged, less virulent form of the virus. Example: Measles, mumps and rubella (MMR) vaccine and the Varicella (chickenpox) vaccine.
- Killed (inactivated) vaccines — killed or inactivated by heat and other methods. Example: Salk's polio vaccine.
- Toxoid vaccines — contain a toxin or chemical secreted by the bacteria or virus. They make us immune to the harmful effects of the infection, instead of to the infection itself. Example: DPT vaccine (Diphtheria, Pertussis and Tetanus).
Second Generation Vaccine contains the pure surface antigen of the pathogen. Example: Hepatitis B vaccine.
Third Generation Vaccine contains the purest and the highest potency vaccines which are synthetic in generation. The latest revolution in vaccine is DNA vaccine or recombinant vaccine.
Do You Know?
Vaccino therapy is the method of use of vaccine for treatment of disease.
Dr. Edward Jenner prepared first vaccine for small pox in 1796.
Polio vaccine was developed by Dr. Jonas Salk (vaccine consists of inactivated microorganism) and Dr. Albert Sabin (live attenuated oral polio vaccine).
Louis Pasteur (1885) discovered vaccine against rabies, anthrax and cholera.
BCG vaccine was developed by Calmette and Guerin against tuberculosis in France in the year 1908.
8.10 Vaccination and Immunization
"Vaccination is the process of administering a vaccine into the body or the act of introducing a vaccine into the body to produce immunity to a specific disease."
Immunization is the process of the body building up immunity to a particular disease. Immunization describes the actual changes in the body after receiving a vaccine. Vaccines work by fighting the pathogen and then recording it in their memory system to ensure that the next time this pathogen enters the body, it is eliminated far quickly. Once, the body is able to fight against the disease, it is believed to have built the immunity for it, also known as the body being immunized against the disease.
8.11 Hypersensitivity
Some of the individuals are very sensitive to some particles present in the environment. The exaggerated response of the immune system to certain antigens present in the environment is called allergy (allo — altered, erg — reaction). The substances to which such an immune response is produced are called allergens. An allergen is an antigen that causes an allergic reaction.
Allergic reactions begin within few seconds after the contact with the allergen and last about half an hour. The common examples of allergens are mites in dust, pollens and some proteins in insect venom. Hay fever and asthma are some common examples of allergy.
Symptoms of allergic reactions include sneezing, watery eyes, running nose and difficulty in breathing. Allergy is a form of over active immune response mediated by immunoglobulin E and mast cells. It can also be due to the release of chemicals like histamine and serotonin from the mast cells.
Anaphylaxis is the classical immediate hypersensitivity reaction. It is a sudden, systematic, severe and immediate hypersensitivity reaction occurring as a result of rapid generalized mast-cell degranulation.
8.12 Immunodeficiency Diseases
Immunodeficiency results from the failure of one or more components of the immune system. Primary immune deficiencies are caused by genetic developmental defects. Secondary immune deficiencies arise due to various reasons like radiation, use of cytolytic and immunosuppressive drugs and infections.
AIDS is an acronym for Acquired Immuno Deficiency Syndrome. It is the deficiency of immune system, acquired during the life time of an individual indicating that it is not a congenital disease. AIDS is caused by Human Immuno Deficiency Virus (HIV). It selectively infects helper T cells. The infected helper T cells will not stimulate antibody production by B cells resulting in loss of natural defence against viral infection.
On the basis of genetic characteristics and differences in the viral antigens, HIV is classified into the types 1 and 2 (HIV-1, HIV-2).
Structure of HIV
The human immunodeficiency virus belongs to the genus Lentivirus. When observed under the electron microscope, HIV is seen as a spherical virus, 100 to 120 nm in diameter, containing a dense core surrounded by a lipoprotein envelope. The envelope has glycoprotein (gp) spikes termed gp41 and gp120. At the core, there are two large single stranded RNA. Attached to the RNA are molecules of reverse transcriptase. It also contains enzymes like protease and ribonuclease. The core is covered by a capsid made of proteins. This is followed by another layer of matrix proteins.
HIV Transmission
The HIV is often located within the cells especially in macrophages. HIV can survive for 1.5 days inside a cell but only about 6 hours outside a cell.
Routes of HIV transmission include:
- Unsafe sexual contact
- Blood-contaminated needles
- Organ transplants
- Blood transfusion
- Vertical transmission from HIV infected mother to child
HIV is not transmitted by insects or by casual contact.
After getting into the body of the person, the virus enters into macrophages where RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into the DNA of host cells and directs the infected cells to produce viral particles. The macrophages continue to produce virus and in this way acts like a HIV factory.
Simultaneously, HIV enters into helper T lymphocytes, replicates and produces progeny viruses. The progeny viruses released in the blood attack other helper T lymphocytes. This is repeated, leading to a progressive decrease in the number of helper T lymphocytes in the body of the infected person.
During this period, the person suffers from bouts of fever, diarrhoea and weight loss. Due to decrease in the number of helper T lymphocytes, the person starts suffering from infections and becomes immune deficient and unable to protect against any infection.
Diagnosis of AIDS
A simple blood test is available that can determine whether the person has been infected with HIV. The ELISA test (Enzyme Linked ImmunoSorbent Assay) detects the presence of HIV antibodies. It is a preliminary test. Western blot test is more reliable and a confirmatory test. It detects the viral core proteins. If both tests detect the presence of the antibodies, the person is considered to be HIV positive.
Prevention of AIDS
AIDS has no cure. Prevention of AIDS is the best option. Measures include:
- Advocating safe sex and promoting regular check-up
- Safe blood for transfusion
- Use of disposable needles
- Use of condoms during sexual contact
- Prevention of drug abuse
- AIDS awareness programmes by NACO (National AIDS Control Organisation), NGOs (Non Governmental Organisations) and WHO
8.13 Autoimmune Diseases
Autoimmunity is due to an abnormal immune response in which the immune system fails to properly distinguish between self and non-self and attacks its own body. Our body produces antibodies (auto antibodies) and cytotoxic T cells that destroy our own tissues. If a disease-state results, it is referred to as auto-immune disease. Thus, autoimmunity is a misdirected immune response.
Autoimmunity is evidenced by the presence of auto antibodies and T cells that are reactive with host antigens. When the cells act as antigens in the same body, they are called autoantigens.
Autoimmune diseases in human can be divided into two broad categories:
- Organ-specific autoimmune diseases: The autoimmune process is directed mostly against one organ. The autoantibodies may block the functions performed by the organs. Examples include Hashimoto's thyroiditis, Graves' disease (thyroid gland) and Addison's disease (adrenal glands).
- Non-organ specific (systemic) autoimmune diseases: Autoimmune activity is widely spread throughout the body. Rheumatoid arthritis and multiple sclerosis are examples for systemic disorder.
8.14 Tumour Immunology
A tumour or neoplasm is a group of cells whose growth has gone unchecked. When a tumour continues to grow and invades healthy tissue, it is called cancer. They spread to other parts of the body from the tumour and give rise to secondary tumour. This is known as metastasis.
Tumour may be benign or malignant depending on its characteristics. Benign or non-cancerous tissues are capable of indefinite growth and do not invade other body parts. In the malignant tumour, the cells grow indefinitely, detach and migrate into healthy surrounding tissues.
In normal cells, cell growth and differentiation is highly controlled and regulated. But in cancer cells, there is breakdown of this regulatory mechanism. Normal cells show a property called contact inhibition, which inhibits uncontrolled growth. Cancer cells do not have this property. As a result, cancerous cells divide continuously giving rise to mass of tissues called tumours.
When a cell undergoes malignant transformation, it acquires new surface antigen and may also lose some normal antigens. These antigens are present on the membranes of malignant cells and they induce an immune response. Both humoral and cellular responses can be observed in malignancy.
Cancer cells can avoid immune detection as they are not foreign bodies but are abnormally functioning body cells. This makes them difficult to treat. The concept of immunological surveillance postulates that the primary function of the immune system is to "seek and destroy" malignant cells that arise by somatic mutation. The efficiency of the surveillance mechanism reduces either as a result of ageing or due to congenital or acquired immunodeficiencies, leads to increased incidence of cancer. The development of tumour represents a lapse in surveillance.
Immunotherapy of Cancer
Immunotherapy also called biological therapy uses substances made by the body or in a laboratory (monoclonal antibodies) to improve or to resist the immune system function. Different approaches have been attempted in the immunotherapy of cancer. Immunotherapy appears to be important in getting rid of the residual malignant cells after the gross tumour has been removed.
The best results in the treatment of cancer is to follow an integrated approach to therapy, combining surgery, radiotherapy, chemotherapy and immunotherapy.
Scope of Immunology
The younger graduates in this field can find number of employment opportunities in Government as well as private hospitals. The scope of the immunology is immunotherapy, microbial immunology, clinical immunology, cellular immunology, allergy and immunology, translational immunology, transplantation immunology, neuro inflammatory disorders, tumour immunology, vaccine immunology, inflammatory disorders, ocular immunology and inflammation.
Table 8.5: Differences between Normal Cell and Cancer Cell
| Normal Cells | Cancer Cells |
| Small, uniformly shaped nuclei. Relatively largely cytoplasmic volume. | Large, variable shaped nuclei. Relatively small cytoplasmic volume. |
| Conformity in cell size and shape. | Variation in cell size and shape. |
| Cells arranged in discrete tissues. | Disorganised arrangement of cells. |
| May possess different cell structures. Normal presentation of cell surface markers. | Loss of normal specialised features. Elevated expression of certain cell markers. |
| Lower levels of dividing cells. Cell tissues clearly demarcated. | Large number of dividing cells. Poorly defined tumour boundaries. |
Summary
- Immunology deals with a study of the immune system. The immune system recognises and eliminates the invaders, and the ability of the body to overcome the pathogen is called immunity.
- Immunity is classified into innate immunity and acquired immunity.
- Acquired immunity is further classified into cell mediated immunity and antibody mediated immunity as its components.
- Acquired immunity may be active or passive immunity.
- Immune response is the body's response to pathogens and it may be primary or secondary.
- The organs involved in the origin, maturation and proliferation of lymphocytes are called lymphoid organs. Thymus, bone marrow are primary lymphoid organs. The secondary lymphoid organs are lymph node, MALT, GALT and BALT.
- An antigen is a substance that is specific to an antibody. An immunogen is a substance capable of initiating an immune response.
- Haptens are substances that are non-immunogenic but can react with the product of a specific immune response.
- Substances that can enhance the immune response to an antigen are called adjuvants.
- An epitope is also known as antigenic determinant and is the active part of the antigen. A paratope is the part of antibody.
- Precipitation, agglutination, neutralization, opsonisation etc., are the different types of antigen and antibody reaction.
- A vaccine is a biological preparation that provides active acquired immunity.
- The malfunctioning of immune system leads to hypersensitivity, immunodeficiency or autoimmune diseases.
- A tumour or neoplasm is a group of cells whose growth has gone unchecked. The best results in the treatment of cancer is achieved by an integrated approach to therapy, surgery, radiotherapy, chemotherapy and immunotherapy.
Evaluation
Exercise MCQ — Choose the Correct Answer
1. Colostrum provides
a) Naturally acquired active immunity b) Naturally acquired passive immunity c) Artificially acquired active immunity d) Artificially acquired passive immunity
2. Paratope is an
A) Antibody binding site on variable regions B) Antibody binding site on heavy regions C) Antigen binding site on variable regions D) Antigen binding site on heavy regions
3. Allergy involves
a) Immunoglobulin E b) Immunoglobulin G c) Immunoglobulin A d) Immunoglobulin M
4. Anaphylactic shock is due to
A) Allergic reaction B) Secretion of toxins C) Secretion of histamines D) All the above
5. Spread of cancerous cells to distant sites is termed as
A) Metastasis B) Oncogenes C) Proto-oncogenes D) Malignant neoplasm
6. AIDS virus has
A) Single stranded RNA B) Double stranded RNA C) Single stranded DNA D) Double stranded DNA
7. All are peripheral lymphoid organs except
A) Lymph nodes B) Spleen C) Mucosa associated lymphoid tissue D) Thymus
8. Which is a macrophage?
A) Monocyte B) Erythrocyte C) Thrombocyte D) Lymphocyte
9. True about interferon is that
A) It is synthetic antiviral agent B) It inhibits viral replication in cells C) It is specific for a particular virus D) It causes infection
10. Cell mediated immunity is carried out by _____ while humoral immunity is mainly carried out by _____
A) B cells or T cells B) Epitopes or antigens C) T cells or B cells D) Antibodies or antigens
11. B cells are activated by
A) Complement B) Antibody C) Interferon D) Antigen
12. In agglutination and precipitation reactions, the antigen is a _____ and _____ respectively
A) Whole cell and soluble molecule B) Soluble molecule and whole cell C) Bacterium and virus D) Protein and Antibody
13. B cells that produce and release large amounts of antibody are called
A) Memory cells B) Basophils C) Plasma cells D) Killer cells
14. Raja is injured and got swelling. The swelling is due to the infection of tissue is an example of
A) Mechanical barrier B) Physiological barrier C) Phagocytosis D) Inflammation Exercise 15. Short Answer
15. Given below are some human organs. Identify one primary and one secondary lymphoid organ. Explain its role.
Liver | Thymus | Stomach | Thyroid | Tonsils
Exercise 16. Short Answer
16. How does saliva act in body defence?
Exercise 17. Short Answer
17. How does immune system work?
Exercise 18. Short Answer
18. Name and explain the type of barriers which involve macrophages.
Exercise 19. Short Answer
19. What are interferons? Mention their role.
Exercise 20. Short Answer
20. List out chemical alarm signals produced during inflammation.
Exercise 21. Differentiate Between
21 (A). Innate immunity and acquired immunity
21 (B). Primary and secondary immune responses
21 (C). Active and passive immunity
21 (D). Humoral and CMI immunity
21 (E). Autoimmune disease and Immunodeficiency disease
Exercise 22. Short Answer
22. Where are B-cells and T cells produced in the human body? How do they differ from each other? Mention any two differences.
Exercise 23. Long Answer
23. Explain the process of replication of retrovirus after it gains entry into the human body.
Exercise 24. Short Answer
24. Why is an antibody molecule represented as H₂L₂?
Exercise 25. Long Answer
25. Explain the structure of immunoglobulin with suitable diagram.
Exercise 26. Short Answer
26. What are the cells involved in innate immune system?
Exercise 27. Short Answer
27. Why is opsonisation efficient in phagocytosis?
Exercise 28. Short Answer
28. What is vaccine? What are its types?
Exercise 29. Short Answer
29. A person is infected by HIV. How will you diagnose for AIDS?
Exercise 30. Long Answer
30. Autoimmunity is a misdirected immune response. Justify.
ICT Corner — Immunology
URL: http://media.hhmi.org/biointeractive/vlabs/immunology/index.html
Procedure:
- Step 1: Type the URL or scan the QR code to open the activity page. Click "To enter the lab" to start the test.
- Step 2: Follow the interactive steps guided by the virtual lab starting from centrifugation and to ELISA.
- Step 3: On the right, find the different headers such as "Diagnosis, Background, Notebook, Glossary and Help" and click to know the virtual procedure happening aside on the left.
- Step 4: Find "Launch Gene Body" on the right bottom of the window, and click it to analyse and understand about the cloning.
- Step 5: Complete this ELISA to know if a patient acquired the disease or not.
Activity ICT Corner — Reflection
After completing the virtual ELISA lab activity, write your observations and conclusions below.
Book-back Exercises — Questions & Answers
Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.
I. Choose the Correct Answer
1. Colostrum provides
a) Naturally acquired active immunity b) Naturally acquired passive immunity c) Artificially acquired active immunity d) Artificially acquired passive immunity
Answer: b) Naturally acquired passive immunity
Explanation: Passive immunity does not require the body to produce antibodies; antibodies are introduced from outside. Colostrum provides ready-made antibodies to the infant, which is naturally acquired and passive in nature.
2. Paratope is an
A) Antibody binding site on variable regions B) Antibody binding site on heavy regions C) Antigen binding site on variable regions D) Antigen binding site on heavy regions
Answer: C) Antigen binding site on variable regions
Explanation: A paratope is the antigen binding site and is a part of an antibody which recognizes and binds to an antigen. The variable (V) regions of the heavy and light chains combine to form the antigen binding site.
3. Allergy involves
a) Immunoglobulin E b) Immunoglobulin G c) Immunoglobulin A d) Immunoglobulin M
Answer: a) Immunoglobulin E
Explanation: Allergy is a form of over active immune response mediated by immunoglobulin E (IgE) and mast cells.
4. Anaphylactic shock is due to
A) Allergic reaction B) Secretion of toxins C) Secretion of histamines D) All the above
Answer: A) Allergic reaction
Explanation: Anaphylaxis is the classical immediate hypersensitivity reaction. It is a sudden, systematic, severe and immediate hypersensitivity reaction occurring as a result of rapid generalized mast-cell degranulation.
5. Spread of cancerous cells to distant sites is termed as
A) Metastasis B) Oncogenes C) Proto-oncogenes D) Malignant neoplasm
Answer: A) Metastasis
Explanation: Cancer cells spread to other parts of the body from the tumour and give rise to secondary tumour. This is known as metastasis.
6. AIDS virus has
A) Single stranded RNA B) Double stranded RNA C) Single stranded DNA D) Double stranded DNA
Answer: A) Single stranded RNA
Explanation: At the core of HIV, there are two large single stranded RNA molecules. Attached to the RNA are molecules of reverse transcriptase.
7. All are peripheral lymphoid organs except
A) Lymph nodes B) Spleen C) Mucosa associated lymphoid tissue D) Thymus
Answer: D) Thymus
Explanation: Thymus is a primary lymphoid organ. Lymph nodes, spleen and MALT are secondary (peripheral) lymphoid organs.
8. Which is a macrophage?
A) Monocyte B) Erythrocyte C) Thrombocyte D) Lymphocyte
Answer: A) Monocyte
Explanation: Monocytes when they mature into large cells, they are called macrophages which perform phagocytosis on any foreign organism.
9. True about interferon is that
A) It is synthetic antiviral agent B) It inhibits viral replication in cells C) It is specific for a particular virus D) It causes infection
Answer: B) It inhibits viral replication in cells
Explanation: Interferons induce antiviral state in the uninfected cells, thereby inhibiting viral replication and spread within the body.
10. Cell mediated immunity is carried out by ___ while humoral immunity is mainly carried out by
A) B cells or T cells B) Epitopes or antigens C) T cells or B cells D) Antibodies or antigens
Answer: C) T cells or B cells
Explanation: Cell mediated immunity is brought about by T cells, macrophages and natural killer cells. Humoral immunity is brought about by B cells with the help of antigen presenting cells and T helper cells.
11. B Cells are activated by
A) Complement B) Antibody C) Interferon D) Antigen
Answer: D) Antigen
Explanation: When receptors on a B cell bind with an antigen, the B cell becomes activated and divides rapidly to produce plasma cells, which in turn produce antibodies.
12. In agglutination and precipitation reactions, the antigen is a ___ and ___ respectively
A) Whole cell and soluble molecule B) Soluble molecule and whole cell C) Bacterium and virus D) Protein and Antibody
Answer: A) Whole cell and soluble molecule
Explanation: In agglutination, a particulate antigen (whole cell) interacts with its antibody causing clumping. In precipitation, a soluble antigen reacts with antibody to form a visible precipitate.
13. B cells that produce and release large amounts of antibody are called
A) Memory cells B) Basophils C) Plasma cells D) Killer cells
Answer: C) Plasma cells
Explanation: When a B cell is activated by an antigen, it divides rapidly to produce plasma cells. The plasma cells produce antibodies. Some B cells do not produce antibodies but become memory cells.
14. Raja is injured and got swelling. The swelling is due to the infection of tissue is an example of
A) Mechanical barrier B) Physiological barrier C) Phagocytosis D) Inflammation
Answer: D) Inflammation
Explanation: Tissue damage and infection induce leakage of vascular fluid, containing chemotactic signals like serotonin, histamine and prostaglandins. They influx phagocytic cells into the affected area — this is the inflammatory barrier response.
II. Answer Briefly
15. Given below are some human organs. Identify one primary and one secondary lymphoid organ. Explain its role.
Liver Thymus Stomach Thyroid Tonsils
Answer: Primary lymphoid organ – Thymus: The thymus is located behind the sternum, above the heart. It is where T lymphocytes mature and become immunocompetent. It secretes the hormone thymosin which stimulates T cells to become mature. It is most active during the neonatal and pre-adolescent periods.
Secondary lymphoid organ – Tonsils: Tonsils are a pair of soft tissue masses located at the back of the throat (pharynx). They are part of the lymphatic system and help fight infections by stopping invading germs including bacteria and viruses.
16. How does saliva act in body defence?
Answer: Saliva contains lysozyme, which acts as an antibacterial agent and cleaves the bacterial cell wall, thus preventing microbial infections. Lysozyme is one of the chemical mediators in the physiological barrier of innate immunity.
17. How does immune system work?
Answer: The immune system recognizes antigens (foreign molecules) and initiates destruction and elimination of invading organisms and toxic molecules produced by them. It has the ability to distinguish foreign molecules from self. It works through innate (non-specific) mechanisms and acquired (specific) mechanisms involving lymphocytes, antibodies and memory cells to protect the body.
18. Name and explain the type of barriers which involve macrophages.
Answer: The barrier that involves macrophages is the phagocytic barrier. Specialized cells such as monocytes, neutrophils and tissue macrophages phagocytose and digest whole microorganisms. Monocytes, when they mature into large cells, are called macrophages and they perform phagocytosis on any foreign organism, thus acting as an important innate defense mechanism.
19. What are interferons? Mention their role.
Answer: Interferons are chemical mediators that form part of the physiological barrier of innate immunity. Their role is to induce an antiviral state in the uninfected cells, thereby preventing the spread of viral infection to healthy cells and protecting the body from viral diseases.
20. List out chemical alarm signals produced during inflammation.
Answer: During inflammation, tissue damage and infection induce leakage of vascular fluid containing chemotactic signals. The chemical alarm signals produced during inflammation are serotonin, histamine and prostaglandins. These signals influx phagocytic cells into the affected area through a process called diapedesis.
21. Differentiate between
(A) Innate immunity and acquired immunity
(B) Primary and secondary immune responses
(C) Active and passive immunity
(D) Humoral and CMI immunity
(E) Autoimmune disease and Immunodeficiency disease
Answer:
(A) Innate vs Acquired Immunity:
Innate immunity is the natural, non-specific resistance present from birth, effective against a wide range of pathogens. Acquired immunity is developed after birth, is antigen-specific, has immunological memory, and can distinguish self from non-self.
(B) Primary vs Secondary Immune Response:
Primary immune response occurs on first contact with an antigen; it is slow and antibody level peaks in 7–10 days. Secondary immune response occurs on re-exposure to the same antigen; it is rapid, antibody level peaks in 3–5 days and remains high for a longer period (booster response).
(C) Active vs Passive Immunity:
Active immunity is produced by the host's own immune system through contact with a pathogen or antigen; it is durable and has immunological memory. Passive immunity is acquired by receiving antibodies from outside; it is transient, has no memory and develops immediately.
(D) Humoral vs Cell Mediated Immunity (CMI):
Humoral immunity involves destruction of pathogens by production of antibodies, brought about by B cells. CMI involves destruction of pathogens by cells without producing antibodies, brought about by T cells, macrophages and natural killer cells.
(E) Autoimmune Disease vs Immunodeficiency Disease:
In autoimmune disease, the immune system fails to distinguish self from non-self and attacks its own body tissues (e.g., Rheumatoid arthritis). In immunodeficiency disease, one or more components of the immune system fail, making the individual unable to fight infections (e.g., AIDS).
22. Where are B-cells and T cells produced in the human body? How do they differ from each other? Mention any two differences.
Answer: Both B and T lymphocytes are produced in the bone marrow. B lymphocytes stay in the bone marrow until they mature, whereas T lymphocytes leave the bone marrow and mature in the thymus gland.
Differences:
1. B cells produce antibodies when activated by antigens, whereas T cells do not produce antibodies but recognize and destroy antigen-presenting cells.
2. B cells are involved in humoral (antibody mediated) immunity, whereas T cells are involved in cell mediated immunity.
23. Explain the process of replication of retrovirus after it gains entry into the human body.
Answer: After HIV (a retrovirus) enters the body, the virus first enters macrophages. Inside the macrophage, the RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into the DNA of host cells and directs the infected cells to produce viral particles. The macrophages continue to produce virus and act like an HIV factory. Simultaneously, HIV enters helper T lymphocytes, replicates and produces progeny viruses. These progeny viruses are released into the blood and attack other helper T lymphocytes, leading to a progressive decrease in their number.
24. Why is an antibody molecule represented as H2L2?
Answer: An antibody molecule is a Y-shaped structure comprising four polypeptide chains: two identical light chains (L) of molecular weight 25,000 Dalton and two identical heavy chains (H) of molecular weight 50,000 Dalton. One light chain is attached to each heavy chain, and two heavy chains are attached to each other, forming a Y-shaped structure. Hence, an antibody is represented as H2L2.
III. Answer in Detail
25. Explain the structure of immunoglobulin with suitable diagram.
Answer: An antibody (immunoglobulin) is a Y-shaped structure composed of four polypeptide chains — two identical light chains (L) of molecular weight 25,000 Dalton (approximately 214 amino acids) and two identical heavy chains (H) of molecular weight 50,000 Dalton (approximately 450 amino acids). The chains are linked by disulphide (S-S) bonds. One light chain is attached to each heavy chain, and two heavy chains are joined to each other, giving the Y shape. Hence, the antibody is represented as H2L2.
Each chain has two terminals: the C terminal (Carboxyl) and the N terminal (Amino). Each chain has two regions: a variable (V) region at one end and a larger constant (C) region at the other end. The V regions of different antibodies vary (to bind different antigens), while the C regions are the same in all antibodies.
In each arm of the Y-shaped monomer, the V regions of the heavy and light chains combine to form an antigen binding site shaped to fit a specific antigenic determinant. Each antibody monomer thus has two antigen binding sites. The heavy chains have a flexible hinge region at their middles. The C regions form the stem of the antibody and determine the antibody class (IgG, IgM, IgA, IgD, IgE) and serve common functions. Functions of immunoglobulins include agglutination, precipitation, opsonisation and neutralization.
26. What are the cells involved in innate immune system?
Answer: The cells involved in the innate immune system include:
1. Monocytes — they mature into macrophages and perform phagocytosis on foreign organisms.
2. Neutrophils — specialized phagocytic cells that engulf and digest microorganisms.
3. Tissue macrophages — phagocytose and digest whole microorganisms.
4. Natural killer cells — involved in cell mediated immune responses.
These cells act as phagocytic and inflammatory barriers. Dendritic cells also participate by presenting antigens to T helper cells. All these cells are derived from pluripotent haematopoietic stem cells.
27. Why is opsonisation efficient in phagocytosis?
Answer: Opsonisation is efficient in phagocytosis because it involves the binding of an opsonin (i.e., antibody) to a receptor on the pathogen's cell membrane, which marks the pathogen for destruction. After opsonin binds to the membrane, phagocytes are attracted to the pathogen. This coating of the pathogen with opsonin makes it easier for phagocytes to recognize, engulf and destroy the pathogen, resulting in a much more efficient phagocytosis compared to uncoated pathogens.
28. What is vaccine? What are its types?
Answer: A vaccine is a biological preparation that provides active acquired immunity to a particular disease. It resembles a disease-causing microorganism and is often made from weakened or attenuated or killed forms of the microbes, their toxins, or one of its surface proteins. Vaccines teach the body how to defend itself without actually experiencing the disease.
Types of Vaccines:
Vaccines are classified as first, second and third generation vaccines.
First Generation:
(i) Live attenuated vaccines — use weakened forms of the virus (e.g., MMR vaccine, Varicella vaccine).
(ii) Killed (inactivated) vaccines — killed by heat or other methods (e.g., Salk's polio vaccine).
(iii) Toxoid vaccines — contain toxins secreted by bacteria/virus, providing immunity to harmful effects (e.g., DPT vaccine).
Second Generation: Contains pure surface antigen of the pathogen (e.g., Hepatitis B vaccine).
Third Generation: Contains synthetic, highest potency vaccines; the latest revolution is the DNA vaccine or recombinant vaccine.
29. A person is infected by HIV. How will you diagnose for AIDS?
Answer: AIDS is diagnosed using two tests:
1. ELISA test (Enzyme Linked ImmunoSorbent Assay) — This is a preliminary test that detects the presence of HIV antibodies in the blood. A simple blood test is available for this purpose.
2. Western Blot test — This is a more reliable and confirmatory test that detects the viral core proteins.
If both tests detect the presence of antibodies, the person is considered to be HIV positive. Clinical symptoms during infection include bouts of fever, diarrhoea and weight loss, along with progressive decrease in helper T lymphocyte count leading to immune deficiency.
30. Autoimmunity is a misdirected immune response. Justify.
Answer: Autoimmunity arises when the immune system fails to properly distinguish between self and non-self and attacks its own body. The body produces auto antibodies and cytotoxic T cells that destroy the body's own tissues. When the body's cells act as antigens in the same body, they are called autoantigens. This misdirected immune response results in autoimmune diseases.
Autoimmune diseases may be organ-specific (e.g., Hashimoto's thyroiditis, Graves' disease, Addison's disease) or systemic (e.g., Rheumatoid
📝 Summary
- Immunology is the study of the immune system, which protects individuals from foreign environmental agents and infective organisms. The overall ability of the body to fight disease-causing pathogens is called immunity, and any substance capable of eliciting an immune response is called an antigen (antibody generator).
- There are two broad classes of immunity: innate immunity (non-specific, natural immunity present from birth) and acquired immunity. Innate immunity operates through anatomical, physiological, phagocytic, and inflammatory barriers to defend against a wide range of microorganisms.
- Innate defense mechanisms include physical barriers like skin (with an acidic pH of 3–5) and mucus membranes, as well as chemical mediators such as lysozyme, interferons, and complement substances that destroy or inhibit pathogens.
- The immune system has the fundamental ability to distinguish foreign molecules from the body's own molecules (self vs. non-self). When this distinction fails, the immune system may attack the host's own molecules, leading to autoimmune diseases, which can be fatal.
- Key topics in immunology include acquired immunity, immune responses, lymphoid organs, antigens, antibodies, antigen-antibody interactions, vaccines, hypersensitivity, immunodeficiency diseases, autoimmune diseases, and tumour immunology.