This MCQ module is based on: Immunity Immune System
TOPIC 31 OF 64
Immunity Immune System
🎓 Class 12
Biology
CBSE
Theory
Ch 7 – Human Health and Disease
⏱ ~14 min
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Immunity Immune System
8.10 Immunity
Every day we are exposed to enormous numbers of infectious agents, yet only a small fraction of these exposures produce disease. The reason is the body's ability to defend itself — the overall capacity of the host to fight disease-causing organisms is called immunity, conferred by the immune system. Immunity is broadly classified into two categories:
- Innate immunity — non-specific defence present from birth.
- Acquired immunity — pathogen-specific defence developing after exposure.
8.11 Innate Immunity
Innate immunity is non-specific and present at birth. It works through four types of barriers:
| Barrier | Components | How it works |
|---|---|---|
| Physical | Skin; mucus coating respiratory, GI, urogenital tracts | Prevents/traps entry of microbes |
| Physiological | Acid in stomach, saliva, tears | Antimicrobial enzymes and pH prevent growth |
| Cellular | Neutrophils (PMN), monocytes, natural killer cells, tissue macrophages | Phagocytose & destroy microbes |
| Cytokine | Interferons secreted by virus-infected cells | Protect surrounding uninfected cells from viral infection |
8.12 Acquired Immunity
Acquired immunity is pathogen-specific and shows memory. The first encounter produces a low-intensity primary response; subsequent encounters with the same pathogen elicit a much stronger secondary or anamnestic response. This is the basis of vaccination.
B and T Lymphocytes
Two special types of lymphocytes carry out acquired immunity:
- B-lymphocytes produce antibodies — proteins released into the blood to fight pathogens.
- T-lymphocytes (T-cells) do not secrete antibodies themselves but help B-cells produce them; they also directly attack infected cells via cell-mediated immunity (CMI).
Two Types of Acquired Immune Response
| Type | Mediated by | Effective against |
|---|---|---|
| Humoral (antibody-mediated) | B-cells, antibodies in blood | Free pathogens in blood/lymph (bacteria, toxins) |
| Cell-mediated (CMI) | T-cells | Intracellular pathogens (viruses, tumor cells), graft rejection |
Cell-mediated immunity is responsible for graft rejection in organ transplantation. The body distinguishes "self" from "nonself" and rejects mismatched tissues. Therefore, tissue matching and blood-group matching are essential before transplants, and patients must take immunosuppressants for life.
8.13 Structure of an Antibody
An antibody molecule (also called immunoglobulin, Ig) has a characteristic Y-shape. It consists of four peptide chains: two long heavy (H) chains and two short light (L) chains. Hence each antibody is represented as H₂L₂.
Different classes of antibodies exist: IgA, IgM, IgE, IgG, IgD — each with different roles.
Five Classes of Antibodies (Immunoglobulins)
| Class | Location | Role |
|---|---|---|
| IgG | Blood — most abundant | Main antibody in secondary response; crosses placenta to protect foetus |
| IgM | Blood; surface of B-cells | First antibody made in primary response; pentameric (5 units) |
| IgA | Mucosal secretions, saliva, tears, breast milk (colostrum) | Mucosal immunity; passes protection to infant via colostrum |
| IgE | Bound to mast cells | Triggers allergic reactions; defends against helminths |
| IgD | B-cell surface | Receptor for B-cell activation |
8.14 Active and Passive Immunity
When a host is exposed to antigens and produces its own antibodies, the immunity is called active immunity — slow to develop but long-lasting.
When ready-made antibodies are directly given to a person, the immunity is called passive immunity — fast-acting but short-lived.
| Type | Source | Onset | Duration | Example |
|---|---|---|---|---|
| Active – Natural | Recovery from natural infection | Slow (1–2 weeks) | Long (years/life) | Immunity after chickenpox |
| Active – Artificial | Vaccination | Slow (1–2 weeks) | Long (years) | Polio, MMR vaccines |
| Passive – Natural | Mother → baby via placenta & colostrum (IgA, IgG) | Immediate | Weeks to months | Newborn immunity |
| Passive – Artificial | Injection of pre-formed antibodies (antiserum) | Immediate | Short (weeks) | Snake antivenom; tetanus antitoxin |
Why is mother's milk essential for newborns? The yellowish fluid colostrum secreted during the initial days of lactation is rich in IgA antibodies that protect the infant during the vulnerable early weeks. The foetus also receives antibodies from the mother through the placenta during pregnancy. Both are examples of natural passive immunity.
8.15 Vaccination and Immunisation
Vaccination exploits the memory property of acquired immunity. In a vaccine, antigenic proteins of a pathogen — or inactivated/weakened pathogen — are introduced into the body. The antibodies produced against these antigens would neutralise the real pathogen during actual infection. Vaccines also generate memory B and T cells that recognise the pathogen quickly on later exposure.
If a person is infected with a deadly microbe and immune response is needed instantly (e.g., tetanus), pre-formed antibodies (antitoxin) are injected directly — this is passive immunisation. Snakebite antivenom is another example.
Recombinant DNA technology allows production of antigenic polypeptides in bacteria or yeast — for example, the hepatitis B vaccine produced from yeast.
8.16 Allergies
An allergy is an exaggerated response of the immune system to certain environmental antigens. The substances that trigger this are called allergens — common ones include dust mites, pollen, animal dander.
Allergic reactions are mediated by IgE antibodies. On allergen exposure, IgE-coated mast cells release histamine and serotonin, producing the typical symptoms: sneezing, watery eyes, running nose, difficulty in breathing.
Treatment: Anti-histamine drugs, adrenalin and steroids quickly reduce allergy symptoms.
Hygiene hypothesis: Modern-day lifestyle (clean indoor environments, antibacterial soaps, reduced outdoor play) may lower normal microbial exposure during early life. The immune system, "under-trained," becomes more sensitive to harmless antigens — explaining the rise of asthma and allergies in metro cities.
8.17 Auto-Immune Disorders
The immune system normally distinguishes "self" from "nonself." Sometimes — due to genetic or unknown reasons — the body attacks its own cells. This results in auto-immune disease. Example: rheumatoid arthritis — the immune system attacks joint tissues, causing chronic inflammation and pain.
8.18 The Immune System in the Body
The human immune system consists of lymphoid organs, tissues, cells, and soluble molecules such as antibodies. Lymphoid organs are the sites where origin, maturation and proliferation of lymphocytes occur.
Primary Lymphoid Organs
Where immature lymphocytes differentiate into antigen-sensitive lymphocytes:
- Bone marrow — main lymphoid organ; all blood cells (including B-lymphocytes) are produced here.
- Thymus — lobed organ located near the heart, beneath the breastbone. Site of T-lymphocyte maturation. Large at birth; shrinks with age, very small after puberty.
Secondary Lymphoid Organs
Where lymphocytes interact with antigens and proliferate into effector cells:
- Spleen — large bean-shaped organ; contains lymphocytes and phagocytes; filters blood by trapping blood-borne microorganisms; also a reservoir of erythrocytes.
- Lymph nodes — small solid structures at various points along the lymphatic system; trap microbes and antigens in lymph and tissue fluid; activate lymphocytes.
- Tonsils, Peyer's patches (of small intestine), appendix.
- MALT (Mucosa-Associated Lymphoid Tissue) — located within the lining of the major tracts (respiratory, digestive, urogenital); constitutes about 50% of lymphoid tissue in the human body.
Interactive: Immunity-Type Identifier
Pick a scenario and learn whether it is active or passive, natural or artificial:
Active or Passive: —
Natural or Artificial: —
Onset / Duration: —
Activity 8.2 — Predict-Observe-Explain: Why Don't Vaccines Cure?
Setup: A doctor explains that the tetanus vaccine is given before any infection (in childhood) but the tetanus antitoxin is given only after a deep wound has been sustained.
Predict: Why are these two preparations used differently? What kind of immunity does each provide?
Explanation:
- Tetanus vaccine = inactivated toxin (toxoid). Gives active artificial immunity. The body takes 1–2 weeks to make its own antibodies — too slow to fight an active tetanus infection. So the vaccine must be given in advance. Once given, protection lasts years and is reinforced by booster doses.
- Tetanus antitoxin = ready-made antibodies from an animal source. Gives passive artificial immunity — acts immediately, exactly what's needed when a deep wound is already at risk. But the antibodies are gone within weeks.
Why both are needed: The vaccine builds long-term defence; the antitoxin gives emergency protection. They complement each other in tetanus management.
Worked Examples
Worked Example 1: Draw and label the structure of an antibody.
Refer to Fig. 8.2.1 above. Key points to label:
- Y-shape overall.
- 2 heavy (H) chains — long, form the stem and partial arms.
- 2 light (L) chains — short, on outer side of the arms.
- Disulphide bonds (S–S) linking H to L and H to H.
- Variable region (V) at tips of arms — antigen-binding sites (2 per antibody).
- Constant region (C) — stem (Fc region).
- Hinge region — flexible joint allowing arms to move.
- Notation: H₂L₂.
Worked Example 2: Differentiate between innate and acquired immunity.
| Feature | Innate | Acquired |
|---|---|---|
| Presence | From birth | Develops after exposure |
| Specificity | Non-specific | Pathogen-specific |
| Memory | None | Yes (anamnestic response) |
| Components | Skin, mucus, stomach acid, NK cells, interferons | B-cells, T-cells, antibodies |
| Speed | Immediate | Slow (1–2 weeks for primary) |
| Examples | Skin barrier; tear lysozyme; phagocytosis | Immunity after chickenpox; vaccine response |
Worked Example 3: Why is the secondary immune response stronger and faster than the primary?
After the first exposure to a pathogen, most of the activated B- and T-cells die. But a small fraction become long-lived memory cells that survive in the body for years or decades.
On second exposure to the same antigen:
On second exposure to the same antigen:
- Memory B-cells immediately recognise the antigen and rapidly divide into plasma cells producing antibodies.
- Antibody production reaches peak in ~3–4 days (vs ~1–2 weeks for primary response).
- Antibody titres are 100–1000× higher than in the primary response.
- Antibodies are predominantly IgG (vs IgM in primary) — higher affinity, longer-lived.
Competency-Based Questions
Q1. The primary lymphoid organs in humans are: L1 Remember
Answer: (b) Bone marrow and thymus. These are where immature lymphocytes differentiate into mature, antigen-sensitive lymphocytes. The other options listed are all secondary lymphoid organs (sites where mature lymphocytes encounter antigens).
Q2. An antibody molecule has: L2 Understand
Answer: (a) H₂L₂. Two heavy and two light chains, linked by disulphide bonds, forming the characteristic Y-shape with two antigen-binding sites at the tips.
Q3. Short Answer: Why is colostrum important for a newborn baby? L2 Understand
Answer: Colostrum — the yellowish fluid secreted during the first few days of lactation — is rich in IgA antibodies. These antibodies coat the infant's gut mucosa and protect against gut infections. The newborn's own immune system is still immature, so this natural passive immunity from the mother is crucial during the first weeks.
Additionally, colostrum contains growth factors, white blood cells, and concentrated nutrients. WHO recommends exclusive breastfeeding for at least the first six months.
Additionally, colostrum contains growth factors, white blood cells, and concentrated nutrients. WHO recommends exclusive breastfeeding for at least the first six months.
Q4. Analyse: Why does a snakebite victim need antivenom serum (not a snake-poison vaccine)? L4 Analyse
Answer: Snake venom can kill within hours; there is no time for the patient's body to make antibodies (which takes 1–2 weeks). The vaccine approach (active immunity) would be too slow.
Antivenom serum contains ready-made antibodies (raised in horses or sheep against the venom). Injecting these into the patient gives passive artificial immunity — the antibodies act immediately, neutralising the venom before it causes irreversible damage. This is the only viable strategy in acute, fast-acting toxicities.
Antivenom serum contains ready-made antibodies (raised in horses or sheep against the venom). Injecting these into the patient gives passive artificial immunity — the antibodies act immediately, neutralising the venom before it causes irreversible damage. This is the only viable strategy in acute, fast-acting toxicities.
Q5. HOT (Create): Hypothesise why the thymus shrinks after puberty. L6 Create
Hypothesis: The thymus is the "school" for T-lymphocyte maturation. Its key job is producing the diverse repertoire of naive T-cells the body will need for life. This task is mostly done during childhood and early adolescence — by puberty, the body has accumulated enough memory and naive T-cells to handle most future infections.
Why shrink? Maintaining the thymus is metabolically costly. After producing a sufficient T-cell repertoire, the organ involutes (shrinks) to conserve energy. Sex hormones at puberty are thought to accelerate this involution.
Consequences: Elderly people have weaker T-cell responses (immunosenescence) — partly because of cumulative thymic involution. This is why vaccines often work less well in old age and why infections like influenza are more dangerous to the elderly.
Research frontier: Could we regenerate the thymus to boost elderly immunity? Active area of research with promising mouse models.
Why shrink? Maintaining the thymus is metabolically costly. After producing a sufficient T-cell repertoire, the organ involutes (shrinks) to conserve energy. Sex hormones at puberty are thought to accelerate this involution.
Consequences: Elderly people have weaker T-cell responses (immunosenescence) — partly because of cumulative thymic involution. This is why vaccines often work less well in old age and why infections like influenza are more dangerous to the elderly.
Research frontier: Could we regenerate the thymus to boost elderly immunity? Active area of research with promising mouse models.
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Innate immunity is non-specific and present from birth.
R: It includes physical, physiological, cellular and cytokine barriers but does not show memory.
Answer: (A). Both true; R explains A. The four-barrier framework defines what innate immunity does and confirms it is non-specific and memory-less.
A: Vaccination produces active immunity.
R: The vaccine provides ready-made antibodies directly to the recipient.
Answer: (C). A is TRUE — vaccines produce active immunity. R is FALSE — vaccines contain weakened or killed pathogens (antigens), not ready-made antibodies. The recipient's own immune system produces antibodies. Ready-made antibodies = passive immunity (e.g., antivenom).
A: Rheumatoid arthritis is an example of an auto-immune disease.
R: In auto-immune diseases, the immune system fails to distinguish self from non-self and attacks the body's own tissues.
Answer: (A). Both true; R explains A. In rheumatoid arthritis, the immune system attacks the synovial membranes of joints — a typical self-attack pattern.
Frequently Asked Questions - Immunity Immune System
What is the main concept covered in Immunity Immune System?
In NCERT Class 12 Biology Chapter on Human Health and Disease, "Immunity Immune System" covers the core biological structures, processes, and pathways students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and physiological/genetic significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Immunity Immune System useful in real-life or applied biology?
Real-life applications of "Immunity Immune System" from NCERT Class 12 Biology Human Health and Disease include medical diagnostics, agriculture, biotechnology, public health, evolutionary insights, and ecological monitoring. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems and real-world challenges.
What are the key terms students should memorize for Immunity Immune System?
Key terms in "Immunity Immune System" (NCERT Class 12 Biology Human Health and Disease) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of the chapter?
NCERT Class 12 Biology Human Health and Disease is structured so each part builds biological understanding sequentially. "Immunity Immune System" connects to neighbouring parts via shared mechanisms, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Immunity Immune System?
CBSE board questions from "Immunity Immune System" typically include: (1) 1-mark MCQs on definitions and processes, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining mechanism + diagram + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Immunity Immune System" lesson allows students to explore biological processes, classifications, or pathways using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function/outcome changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
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