GS Paper III · Science & Technology · Biology · Class XII NCERT
Immunity & the Human Immune System
Complete UPSC notes — Lymphoid Organs, Immune Cells, Antibodies, Innate vs Acquired Immunity, Active vs Passive Immunity, Vaccination — with all 7 diagrams, PYQs, MCQs and Current Affairs.
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Overview — What is Immunity?
Definition · Components · How the immune system works
Definition
Immunity is the ability of an organism to defend itself from the harmful effects of microorganisms (pathogens) and their byproducts. The immune system can identify foreign antigens, react to them, and retain memory of them for the future.
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Antigen
Any foreign substance (protein, polysaccharide, lipid, nucleic acid) that triggers an immune response. Usually on the surface of pathogens. "Anti" = against + "Gen" = generator.
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Antibody
Protein (immunoglobulin) produced by plasma B cells specifically against an antigen. Binds to antigen like a lock-and-key. Part of humoral immunity.
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Immunological Memory
After first exposure to antigen, memory B and T cells are formed. On re-exposure → massive, rapid secondary response. Basis of vaccination.
Three Components of the Immune System
1. Lymphoid organs — where immune cells originate, mature, and are deployed (thymus, bone marrow, spleen, lymph nodes, tonsils, MALT)
2. Immune cells — WBCs: lymphocytes (B & T cells), neutrophils, macrophages, natural killers, monocytes
3. Soluble molecules — Antibodies (immunoglobulins), cytokines, interferons, complement proteins
2. Immune cells — WBCs: lymphocytes (B & T cells), neutrophils, macrophages, natural killers, monocytes
3. Soluble molecules — Antibodies (immunoglobulins), cytokines, interferons, complement proteins
| Feature | Innate Immunity | Acquired (Adaptive) Immunity |
|---|---|---|
| Present from | Birth (non-specific) | Developed after exposure to antigen |
| Specificity | Non-specific — acts on all pathogens | Pathogen-specific — tailored response |
| Memory | No immunological memory | Yes — memory B and T cells formed |
| Speed | Immediate (within minutes–hours) | Slower (days) first time; fast on re-exposure |
| Key cells | Neutrophils, Macrophages, NK cells | B lymphocytes, T lymphocytes |
| Key molecules | Interferons, complement proteins | Antibodies (immunoglobulins) |
| Examples | Skin barrier, stomach acid, phagocytosis, fever | Antibody production, CTL killing, vaccination |
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Lymphoid Organs — Where Immune Cells Are Made
Primary organs (maturation) · Secondary organs (action) · MALT
The Lymphoid System. Left: Full body showing all lymphoid organs — Bone Marrow (in long bones — ALL blood cells including B/T cell precursors originate here), Thymus (above heart — T cell maturation), Spleen (left upper abdomen — filters blood, destroys old RBCs, houses lymphocytes), Lymph Nodes (distributed along lymph vessels — trap antigens from tissue fluid), Lymph Vessels (carry lymph fluid). Right: Close-up of a lymph node (bean-shaped, internal compartments) and the spleen (large, dark-red organ). Primary organs = Bone marrow + Thymus. All others = Secondary lymphoid organs.
Primary vs Secondary Lymphoid Organs — The Key Distinction
Primary lymphoid organs: Where lymphocytes originate and mature into antigen-sensitive cells.
→ Bone marrow (all blood cells formed here; B lymphocytes mature here)
→ Thymus (T lymphocytes mature here)
Secondary lymphoid organs: Where mature lymphocytes interact with antigens and mount an immune response.
→ Spleen, Lymph nodes, Tonsils, Peyer's patches (intestine), MALT
→ Bone marrow (all blood cells formed here; B lymphocytes mature here)
→ Thymus (T lymphocytes mature here)
Secondary lymphoid organs: Where mature lymphocytes interact with antigens and mount an immune response.
→ Spleen, Lymph nodes, Tonsils, Peyer's patches (intestine), MALT
| Organ | Type | Location | Key Functions |
|---|---|---|---|
| Bone Marrow | Primary | Hollow cavities of long bones | Produces ALL blood cells (haematopoiesis). B lymphocytes mature here. T cell precursors produced here, then migrate to thymus. |
| Thymus | Primary | Above the heart (mediastinum) | T lymphocytes mature here. Produces thymosin hormone (promotes T cell maturation). Largest in infancy; shrinks with age (involutes after puberty). Very important for UPSC |
| Spleen | Secondary | Left upper abdomen | Bean-shaped. Filters blood; destroys old/damaged RBCs; houses lymphocytes and phagocytes; interacts with blood-borne antigens. Called "graveyard of RBCs." |
| Lymph Nodes | Secondary | Along lymph vessels (neck, armpit, groin) | Small, bean-shaped structures. Trap antigens from tissue fluid and lymph. Activate lymphocytes. Swell during infection (lymphadenopathy). |
| Tonsils | Secondary | Back of throat | First line of defence against inhaled/ingested pathogens. Part of MALT. |
| Peyer's Patches | Secondary | Wall of small intestine (ileum) | Monitor intestinal bacteria; protect against gut pathogens. Part of MALT. |
| MALT (Mucosa-Associated Lymphoid Tissue) | Secondary | Lining of digestive, respiratory, urogenital tracts | ~50% of all lymphoid tissue in the human body. Guards mucosal surfaces — major entry points for pathogens. Produces secretory IgA. |
Mnemonic — Primary Lymphoid Organs
Bone marrow = B cells mature here
Thymus = T cells mature here
Simple: B goes to Bone, T goes to Thymus
Thymus = T cells mature here
Simple: B goes to Bone, T goes to Thymus
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Cells of the Immune System
B cells · T cells · Neutrophils · Macrophages · NK cells · Origin from stem cells
All Blood Cells Arise from ONE Stem Cell. The Haemocytoblast (pluripotent stem cell) in bone marrow gives rise to every type of blood cell. Left branch: Lymphoblasts → B lymphocytes (→ Plasma cells that make antibodies) and T lymphocytes. Middle: Monoblast → Monocyte → Macrophage. Myeloblast → Eosinophil, Basophil, Neutrophil (major innate immune cells). Right: Megakaryoblast → Thrombocytes (platelets); Proerythroblast → Erythrocytes (RBCs). Key UPSC point: All WBCs = immune cells; all originate from bone marrow. The pink area at the bottom shows the lymphocyte family (white blood cells of adaptive immunity).
🔵 T Lymphocytes — Cell-Mediated Immunity
T-Cell Development in the Thymus. T cell precursors from Bone Marrow migrate to the Thymus. Inside the thymus lobule (cortex), they pass through 4 double-negative (DN) stages: DN1→DN2→DN3→DN4. At the DN2 stage, CD25 receptors appear. By DN4, cells become double-positive (DP), expressing both CD4 and CD8 markers. In the medulla, cells undergo selection: CD4+ cells become Helper T cells (TH), CD8+ cells become Cytotoxic T cells (TC). This is thymic selection — ensures T cells respond to foreign antigens without attacking the body's own cells.
Origin: Bone marrow (precursors) → mature in Thymus
Surface marker: T-Cell Receptor (TCR)
Function: Cell-Mediated Immunity (CMI)
Helper T cells (TH / CD4+):
→ Express CD4 on surface
→ Central coordinator of adaptive immunity
→ Activate macrophages (phagocytosis), cytotoxic T cells (killing pathogens), and B cells (antibody production)
→ Think of TH as the "General" of the immune army
Surface marker: T-Cell Receptor (TCR)
Function: Cell-Mediated Immunity (CMI)
Helper T cells (TH / CD4+):
→ Express CD4 on surface
→ Central coordinator of adaptive immunity
→ Activate macrophages (phagocytosis), cytotoxic T cells (killing pathogens), and B cells (antibody production)
→ Think of TH as the "General" of the immune army
Cytotoxic T cells (TC / CD8+):
→ Express CD8 on surface
→ Directly kill intracellular pathogens (viruses inside cells) and cancer cells
→ Release perforin and granzymes to puncture and destroy infected cells
Suppressor T cells (TS):
→ Suppress both humoral and cell-mediated immune responses
→ Prevent autoimmune reactions (attacking own cells)
→ Also called Regulatory T cells (Tregs)
→ Express CD8 on surface
→ Directly kill intracellular pathogens (viruses inside cells) and cancer cells
→ Release perforin and granzymes to puncture and destroy infected cells
Suppressor T cells (TS):
→ Suppress both humoral and cell-mediated immune responses
→ Prevent autoimmune reactions (attacking own cells)
→ Also called Regulatory T cells (Tregs)
🟢 B Lymphocytes — Humoral Immunity (Antibody Production)
B Lymphocyte with B Cell Receptor (BCR). The B cell surface is studded with BCR (B Cell Receptors) — actually membrane-bound antibodies (IgM/IgD monomers). When an antigen binds to the BCR (top right — orange ball = antigen), it triggers B cell activation. Each B cell makes ONE unique BCR that recognises ONE specific antigen. After activation by TH cells: B cell divides into → Plasma cells (antibody factories) + Memory B cells (long-lived; provide future immunity). The colourful receptors on the left represent different surface molecules (CD markers, co-receptors) that help with signalling.
Origin & maturation: Bone marrow (origin AND maturation — unlike T cells)
Surface markers: B Cell Receptor (BCR = membrane-bound antibody); CD19, CD20
First antibodies: IgM and IgD on surface as BCRs (not yet secreted)
Each B cell makes ONE unique antibody with ONE antigen-binding site
Surface markers: B Cell Receptor (BCR = membrane-bound antibody); CD19, CD20
First antibodies: IgM and IgD on surface as BCRs (not yet secreted)
Each B cell makes ONE unique antibody with ONE antigen-binding site
After activation by TH cells:
→ Plasma cells — antibody-secreting factories (live for days–weeks; produce millions of antibodies)
→ Memory B cells — long-lived (years to lifetime); basis of immunological memory
B cell response = Humoral Immune Response (antibodies in blood/lymph)
Also act as APCs (Antigen-Presenting Cells) to TH cells
→ Plasma cells — antibody-secreting factories (live for days–weeks; produce millions of antibodies)
→ Memory B cells — long-lived (years to lifetime); basis of immunological memory
B cell response = Humoral Immune Response (antibodies in blood/lymph)
Also act as APCs (Antigen-Presenting Cells) to TH cells
🟠 Macrophages — The Multi-Function Innate Immune Cell
Monocyte → Macrophage Differentiation. Circulatory Monocytes (small, with kidney-shaped nucleus) circulate in blood. When they migrate into tissues and are stimulated by cytokines, chemokines, TLR ligands, oxidative stress, etc., they differentiate into Macrophages (large, irregular shape, more granules). Macrophage classical functions: (1) Cytotoxicity & Fibrosis — kill pathogens, tumour cells, trigger tissue repair. (2) Phagocytosis, Chemokine & Cytokine production, Tissue surveillance — engulf and destroy pathogens, release chemical signals, continuously patrol tissues. (3) Antigen Presentation & T-Cell Activation — process antigens and present them to TH cells (key bridge between innate and adaptive immunity).
Origin: Myeloid progenitor → Monocyte (blood) → Macrophage (tissues)
Type: Innate immune system cells (but bridge to adaptive immunity)
Functions:
1. Phagocytosis — engulf and destroy pathogens using lysosomes
2. Release cytotoxic proteins — kill tumour cells, intracellular bacteria, virus-infected cells
3. Antigen Presenting Cell (APC) — present antigens to TH cells via MHC complex
4. Activated by TH cells in a feedback loop
Type: Innate immune system cells (but bridge to adaptive immunity)
Functions:
1. Phagocytosis — engulf and destroy pathogens using lysosomes
2. Release cytotoxic proteins — kill tumour cells, intracellular bacteria, virus-infected cells
3. Antigen Presenting Cell (APC) — present antigens to TH cells via MHC complex
4. Activated by TH cells in a feedback loop
Neutrophils:
→ 50–70% of circulating WBCs
→ First cells to arrive at infection site (innate immune response)
→ Short-lived; die after killing pathogens
→ Also phagocytic
Natural Killer (NK) cells:
→ Lymphocytes of the innate immune system (not B or T cells)
→ Kill virus-infected cells and cancer cells WITHOUT needing antigen presentation
→ Kill cells that have lost MHC class I molecules (which cancer cells often do)
→ 50–70% of circulating WBCs
→ First cells to arrive at infection site (innate immune response)
→ Short-lived; die after killing pathogens
→ Also phagocytic
Natural Killer (NK) cells:
→ Lymphocytes of the innate immune system (not B or T cells)
→ Kill virus-infected cells and cancer cells WITHOUT needing antigen presentation
→ Kill cells that have lost MHC class I molecules (which cancer cells often do)
The Complete Adaptive Immune Response — From Microbe to Immunity. This is the most important diagram in immunology for UPSC. Step-by-step: ① Macrophage engulfs microbe (antigen from microbe = nonself molecule). ② Microbe is processed internally; fragments combined with self protein (MHC complex). ③ Self-nonself complex presented on macrophage surface = Antigen-Presenting Cell (APC). ④ T cell receptor (TCR) on Helper T cell recognises the complex. Interleukin-1 secreted by APC stimulates the Helper T cell. ⑤ Helper T cell is activated and secretes Interleukin-2. ⑥ Interleukin-2 activates B cells → Humoral Immunity (plasma cells secrete antibodies). ⑦ Interleukin-2 activates Cytotoxic T cells → Cell-mediated Immunity (attack infected cells directly). This diagram shows why Helper T cells (CD4+) are the "central coordinator" of ALL adaptive immunity.
📋 PYQ — UPSC Prelims2022
Which one of the following statements best describes the role of B cells and T cells in the human body?
- (a) They protect the body from environmental allergens
- (b) They alleviate the body's pain and inflammation
- (c) They act as immunosuppressants in the body
- (d) They protect the body from the diseases caused by pathogens ✓ Correct
Explanation: Both B cells (B lymphocytes) and T cells (T lymphocytes) are the core cells of the adaptive immune system. B cells produce antibodies (humoral immunity) that neutralise pathogens in blood and lymph. T cells mediate cell-mediated immunity — Helper T cells coordinate the immune response, Cytotoxic T cells directly kill pathogen-infected cells and cancer cells. Together, they protect the body from diseases caused by pathogens (bacteria, viruses, fungi, parasites). Option (a) is wrong — allergens trigger IgE, not mainly B/T cells. Options (b) and (c) describe other functions.
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Immunoglobulins (Antibodies) — Structure & Types
Y-shaped structure · IgG · IgM · IgA · IgE · IgD · Applications
Structure of an Immunoglobulin (Antibody) — Y-Shaped. Every antibody has this same basic architecture: Two identical Heavy (H) chains (purple — longer polypeptides) + Two identical Light (L) chains (yellow — shorter). All four chains are linked by Disulfide bonds (S-S). Each chain has two regions: Variable domain (VH and VL — shown at top — the unique antigen-binding region, different for every antibody) + Constant domain (CH1, CH2, CH3, CL — same within each class; determines the class of Ig). The two antigen-binding sites are at the top of each arm (circled, dashed). The hinge region (middle) allows flexibility. The Fc/effector region (bottom) determines which cells the antibody can bind to (macrophages, complement). Carbohydrate groups are attached to CH2. Each basic antibody is bivalent — can bind two identical antigens.
Basic Structure (Must-Know)
All immunoglobulins have: 2 heavy chains + 2 light chains (joined by disulphide bonds) in a Y-shape.
Variable domains (VH + VL) = antigen-binding sites (unique for each antibody).
Constant domains = determine the class (IgG, IgM, IgA, IgE, IgD).
The simplest antibody (monomer) is bivalent — 2 antigen-binding sites.
Variable domains (VH + VL) = antigen-binding sites (unique for each antibody).
Constant domains = determine the class (IgG, IgM, IgA, IgE, IgD).
The simplest antibody (monomer) is bivalent — 2 antigen-binding sites.
| Class | Structure | Key Functions | UPSC Points |
|---|---|---|---|
| IgG | Monomer (bivalent) | • Dominant in secondary immune response • Most abundant Ig in blood (~75%) • Activates macrophages and neutrophils • Fixes complement | ONLY Ig that crosses the placenta → protects newborn. Also found in breast milk. Major therapeutic antibody. |
| IgM | Pentamer (10 antigen-binding sites!) | • Dominates primary immune response • Most effective complement-fixing Ig • Potent agglutinin (clumps antigens) | First antibody produced. Monomer of IgM used as BCR (along with IgD). Largest Ig. ABO blood group antibodies are IgM. |
| IgA | Monomer (serum) or Dimer (secretory) | • Secretory IgA in mucosal secretions (saliva, tears, breast milk, gut lining) • Prevents antigen entry into blood | Most abundant Ig in mother's milk (colostrum). Primary defence at mucosal surfaces. ~15% of serum Ig. |
| IgE | Monomer | • Causes allergic reactions (binds to mast cells → histamine release) • Protection against parasites (helminths) | Lowest concentration in serum. Important in asthma, hay fever, food allergies. High IgE = allergic disease or parasite infection. CA Relevant |
| IgD | Monomer | • Present in very small amounts in serum • Function against pathogens unknown • Part of BCR | IgD may play role in antigen-triggered lymphocyte differentiation. Mostly serves as BCR on mature naive B cells (along with IgM). |
Mnemonic — The 5 Immunoglobulins
GAMED → IgG, IgA, IgM, IgE, IgD
Or remember by order of abundance in serum: G > A > M > D > E
IgG = most abundant (75%); IgE = least abundant but most dramatic (allergies)
Or remember by order of abundance in serum: G > A > M > D > E
IgG = most abundant (75%); IgE = least abundant but most dramatic (allergies)
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Monoclonal Antibodies
Produced by a single clone of plasma B cells. Homogeneous — recognise ONE specific epitope. Require hybridoma cell lines (B cell + myeloma cell fusion). Used in cancer therapy (Herceptin), COVID treatment (Regeneron), diagnostics. High Yield CA
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Polyclonal Antibodies
Produced by different clones of plasma B cells. Heterogeneous mixture — recognise multiple epitopes on same antigen. No hybridoma needed. Used in research. More natural immune response is polyclonal.
📋 PYQ — UPSC PrelimsClassic
Which type of immunoglobulin can cross the placenta and provide immunity to the foetus?
- (a) IgA
- (b) IgM
- (c) IgG ✓ Correct
- (d) IgE
Explanation: IgG is the ONLY immunoglobulin that can cross the human placenta from mother to foetus. This is a specialised process mediated by Fc receptors on placental cells (FcRn receptors). This transferred IgG provides passive immunity to the newborn in its first 3–6 months of life, protecting it against pathogens the mother has been exposed to. IgG is also found in breast milk. IgA is found in colostrum/breast milk (secretory IgA — provides mucosal protection) but does NOT cross the placenta. IgM is the largest Ig (pentamer) and cannot cross the placenta.
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Types of Immunity — Innate, Acquired, Active, Passive
Barriers · Primary vs Secondary Response · Humoral vs Cell-mediated
🔴 Innate Immunity — The First Line of Defence
Non-specific — acts against ALL foreign agents
Present from birth — no prior exposure needed
4 types of barriers:
1. Physical barriers:
→ Skin — primary barrier; mechanical block
→ Mucus lining gut, respiratory, urogenital tracts — traps microbes
→ Cilia in respiratory tract — sweeps microbes out
2. Physiological barriers:
→ Saliva (lysozyme enzyme kills bacteria)
→ Tears (lysozyme)
→ Stomach acid (HCl, pH 2 — kills most pathogens)
→ Human milk — contains antimicrobial factors
→ Fever — high temperature inhibits pathogen growth
Present from birth — no prior exposure needed
4 types of barriers:
1. Physical barriers:
→ Skin — primary barrier; mechanical block
→ Mucus lining gut, respiratory, urogenital tracts — traps microbes
→ Cilia in respiratory tract — sweeps microbes out
2. Physiological barriers:
→ Saliva (lysozyme enzyme kills bacteria)
→ Tears (lysozyme)
→ Stomach acid (HCl, pH 2 — kills most pathogens)
→ Human milk — contains antimicrobial factors
→ Fever — high temperature inhibits pathogen growth
3. Cellular barriers:
→ Neutrophils (PMNL) — first to arrive; phagocytose pathogens
→ Macrophages — phagocytosis + APC
→ Monocytes — precursors to macrophages
→ Natural Killer (NK) cells — kill virus-infected and cancer cells without antigen presentation
4. Cytokine barriers:
→ Interferons — proteins secreted by virus-infected cells that PROTECT surrounding uninfected cells from viral infection
→ Complement system — cascade of proteins that punch holes in bacterial membranes and tag pathogens for destruction
→ Neutrophils (PMNL) — first to arrive; phagocytose pathogens
→ Macrophages — phagocytosis + APC
→ Monocytes — precursors to macrophages
→ Natural Killer (NK) cells — kill virus-infected and cancer cells without antigen presentation
4. Cytokine barriers:
→ Interferons — proteins secreted by virus-infected cells that PROTECT surrounding uninfected cells from viral infection
→ Complement system — cascade of proteins that punch holes in bacterial membranes and tag pathogens for destruction
🟢 Acquired (Adaptive) Immunity — Specific & Memorised
Pathogen-specific — tailored to each antigen
Has memory — stronger response on re-exposure
Primary response: First exposure to antigen → slow, low-intensity response (days). Body is "learning." Produces memory cells.
Secondary response: Re-exposure to same antigen → rapid, high-intensity response (hours). Memory B and T cells recognise antigen immediately → massive antibody production. This is why vaccines work!
Humoral Immune Response (HIR):
→ B cells + plasma cells + antibodies in blood/lymph
→ Protects against extracellular pathogens
Has memory — stronger response on re-exposure
Primary response: First exposure to antigen → slow, low-intensity response (days). Body is "learning." Produces memory cells.
Secondary response: Re-exposure to same antigen → rapid, high-intensity response (hours). Memory B and T cells recognise antigen immediately → massive antibody production. This is why vaccines work!
Humoral Immune Response (HIR):
→ B cells + plasma cells + antibodies in blood/lymph
→ Protects against extracellular pathogens
Cell-Mediated Immunity (CMI):
→ T lymphocytes (especially CD8+ cytotoxic T cells)
→ Kills intracellular pathogens (viruses inside cells)
→ Kills cancer cells
→ TH cells activate macrophages and cytotoxic T cells
→ Responsible for graft rejection (organ transplant rejection)
Key process:
Macrophage (APC) presents antigen via MHC → TH cell activated → Interleukin-2 released → Activates B cells (humoral) AND Cytotoxic T cells (CMI)
→ T lymphocytes (especially CD8+ cytotoxic T cells)
→ Kills intracellular pathogens (viruses inside cells)
→ Kills cancer cells
→ TH cells activate macrophages and cytotoxic T cells
→ Responsible for graft rejection (organ transplant rejection)
Key process:
Macrophage (APC) presents antigen via MHC → TH cell activated → Interleukin-2 released → Activates B cells (humoral) AND Cytotoxic T cells (CMI)
| Feature | Active Immunity | Passive Immunity |
|---|---|---|
| Definition | Antibodies produced by body itself in response to antigen exposure | Readymade antibodies transferred from outside (another individual/source) |
| How induced | Natural infection OR vaccination (intentional injection of antigen) | Mother to foetus (placenta, IgG), Mother to newborn (colostrum, IgA), Anti-serum injection |
| Memory | Yes — memory B and T cells formed → long-lasting | No — no memory cells formed → short-term |
| Onset | Slow (days to weeks) to develop fully | Immediate protection |
| Duration | Long-lasting (months to lifetime) | Short-lived (weeks to months) |
| Examples | Recovery from infection, COVID vaccine, flu vaccine, BCG vaccine | Maternal IgG via placenta, IgA via colostrum, Anti-rabies serum, Anti-snake venom serum (ASV) |
| UPSC note | Basis of vaccination programmes | Anti-snake venom = passive immunity; colostrum = passive immunity for newborn CA |
Colostrum — Passive Immunity for Newborns (NCERT Important)
The yellowish fluid secreted by the mother in the first few days of lactation. Rich in IgA antibodies (secretory IgA). Provides passive immunity to the newborn. It is the reason WHO and NCERT emphasise exclusive breastfeeding for the first 6 months — it literally transfers the mother's antibodies to the baby. IgG also transferred via placenta during pregnancy (before birth). Together: newborns get passive protection from day one.
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Vaccination & Immunisation
Principle · Types of vaccines · India's UIP · Recent developments
Principle of Vaccination
Vaccination exploits immunological memory. A vaccine introduces a harmless form of the antigen (attenuated/killed pathogen, antigen proteins, or genetic instructions to make them) → body mounts a primary immune response → produces memory B and T cells → on actual infection, the secondary response is massive and rapid → pathogen is neutralised before symptoms appear.
| Type of Vaccine | Description | Examples |
|---|---|---|
| Live-attenuated | Weakened (attenuated) live pathogen. Closest to natural infection — strong immune response. Cannot be given to immunocompromised. | BCG (TB), MMR (measles-mumps-rubella), Oral Polio Vaccine (OPV), Yellow fever, Varicella |
| Inactivated/Killed | Killed pathogen. Safer but weaker response; needs boosters. | Covaxin (India's COVID vaccine), IPV (Inactivated Polio), Hepatitis A, Flu vaccine (some) |
| Subunit/Protein | Only specific proteins (antigens) of the pathogen — not the whole pathogen. Very safe. | Hepatitis B vaccine, HPV vaccine (Gardasil/Cervarix), Pertussis component in DPT |
| Toxoid | Inactivated bacterial toxins. Train body to neutralise the toxin, not the bacteria itself. | Tetanus toxoid, Diphtheria toxoid (in DPT vaccine) |
| mRNA vaccines | mRNA encoding the antigen — body's cells produce antigen temporarily → immune response. Novel platform. Nobel Prize 2023 | Pfizer-BioNTech (Comirnaty), Moderna (Spikevax) — COVID-19 vaccines. India developing mRNA vaccine (HGCo19). |
| Viral vector | Another harmless virus (vector) carries antigen gene into body's cells. | Covishield (AstraZeneca/Serum Institute — Oxford), Sputnik V (Russia) — COVID vaccines |
🇮🇳 India's Universal Immunisation Programme (UIP)
UIP is one of the largest public health programmes in the world. Launched: 1985 (expanded from EPI). Provides free vaccines to all children and pregnant women.
Key vaccines under UIP: BCG (TB), OPV (polio), Hepatitis B, DPT (diphtheria, pertussis, tetanus), Hib (Haemophilus influenzae), Measles-Rubella (MR), Japanese Encephalitis, Rotavirus, Pneumococcal Conjugate Vaccine (PCV), IPV, Tetanus-diphtheria (Td) for pregnant women.
Mission Indradhanush — 2015 onwards; targets unimmunised/partially immunised children, especially in hard-to-reach areas.
Key vaccines under UIP: BCG (TB), OPV (polio), Hepatitis B, DPT (diphtheria, pertussis, tetanus), Hib (Haemophilus influenzae), Measles-Rubella (MR), Japanese Encephalitis, Rotavirus, Pneumococcal Conjugate Vaccine (PCV), IPV, Tetanus-diphtheria (Td) for pregnant women.
Mission Indradhanush — 2015 onwards; targets unimmunised/partially immunised children, especially in hard-to-reach areas.
Pulse Polio Programme — India declared Polio-free in 2014 (WHO certification). Uses OPV on National Immunisation Days.
India's COVID-19 vaccination: World's largest vaccination drive. Vaccines used: Covishield, Covaxin, Sputnik V, Corbevax, ZyCov-D (DNA vaccine — world's first approved DNA vaccine), iNCOVACC (intranasal vaccine — world's first approved intranasal COVID vaccine). High Yield CA
CERVAVAC — India's first indigenously developed HPV vaccine (by Serum Institute), approved 2022. Prevents cervical cancer. CA
India's COVID-19 vaccination: World's largest vaccination drive. Vaccines used: Covishield, Covaxin, Sputnik V, Corbevax, ZyCov-D (DNA vaccine — world's first approved DNA vaccine), iNCOVACC (intranasal vaccine — world's first approved intranasal COVID vaccine). High Yield CA
CERVAVAC — India's first indigenously developed HPV vaccine (by Serum Institute), approved 2022. Prevents cervical cancer. CA
📋 PYQ — UPSC Prelims2020
In the context of vaccines, what is "immune memory"?
- (a) The ability of the immune system to remember the first vaccine injected
- (b) The long-term ability of the immune system to recognise and mount a rapid response to a pathogen it has encountered before ✓ Correct
- (c) The ability of lymphocytes to retain the genetic code of antibodies
- (d) The capacity of vaccines to cause long-lasting side effects
Explanation: Immune memory is the capacity of the adaptive immune system to mount a faster, stronger, and more effective response on second exposure to a pathogen (or antigen). After the first exposure (primary response), long-lived Memory B cells and Memory T cells are formed. These cells persist for years to a lifetime. Upon re-exposure to the same antigen, these memory cells are rapidly activated → producing large amounts of antibodies very quickly (secondary response) — often before the pathogen can cause disease. This is the entire biological basis of vaccination: we give the antigen in a harmless form, allow memory cells to form, so that the real pathogen is repelled rapidly on actual encounter.
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Current Affairs — Immunity & Immunology
Nobel Prizes · COVID · Monoclonal antibodies · Autoimmunity · UPSC 2025–2026
🏆 Nobel Prizes in Immunology (UPSC Favourites)
Nobel 2023 (Physiology/Medicine): Katalin Karikó & Drew Weissman — for modified mRNA technology that enabled COVID-19 mRNA vaccines. Modified nucleoside substitution prevents immune system from rejecting mRNA before it can produce the antigen. Foundation of Pfizer-BioNTech and Moderna vaccines.
Nobel 2018 (Physiology/Medicine): James P. Allison & Tasuku Honjo — cancer immunotherapy via immune checkpoint inhibition. Allison: CTLA-4 antibody (blocks immune brake); Honjo: PD-1 antibody (removes another brake). Unleashing the immune system to fight cancer.
Nobel 2011: Bruce Beutler & Jules Hoffmann — innate immunity mechanisms (Toll-like receptors, TLRs). Ralph Steinman — dendritic cells (APCs).
Nobel 1984: César Milstein & Georges Köhler — monoclonal antibody production via hybridoma technology.
Nobel 2018 (Physiology/Medicine): James P. Allison & Tasuku Honjo — cancer immunotherapy via immune checkpoint inhibition. Allison: CTLA-4 antibody (blocks immune brake); Honjo: PD-1 antibody (removes another brake). Unleashing the immune system to fight cancer.
Nobel 2011: Bruce Beutler & Jules Hoffmann — innate immunity mechanisms (Toll-like receptors, TLRs). Ralph Steinman — dendritic cells (APCs).
Nobel 1984: César Milstein & Georges Köhler — monoclonal antibody production via hybridoma technology.
🔬 Key Science Developments (2023–2025)
Monoclonal Antibodies in therapy: Used for cancer (Herceptin for breast cancer, Rituximab for lymphoma), autoimmune diseases (Adalimumab for rheumatoid arthritis), COVID-19 (Bebtelovimab), and more. Growing rapidly in India's pharma sector.
ZyCov-D — World's first DNA vaccine: India approved Zydus Cadila's ZyCov-D for COVID-19 — world's first approved DNA vaccine. Needle-free administration via PharmaJet device.
iNCOVACC — World's first intranasal COVID vaccine: Developed by Bharat Biotech + Washington University. Nasal spray → mucosal immunity (secretory IgA). India first to approve.
CERVAVAC — India's HPV vaccine: Serum Institute. For cervical cancer prevention. India's government providing free HPV vaccination for girls 9–14 years (Union Budget 2023).
Long COVID & immune dysregulation: Research shows persistent immune activation, autoantibodies, and T cell exhaustion in Long COVID patients — major area of immunological research 2023–2025.
ZyCov-D — World's first DNA vaccine: India approved Zydus Cadila's ZyCov-D for COVID-19 — world's first approved DNA vaccine. Needle-free administration via PharmaJet device.
iNCOVACC — World's first intranasal COVID vaccine: Developed by Bharat Biotech + Washington University. Nasal spray → mucosal immunity (secretory IgA). India first to approve.
CERVAVAC — India's HPV vaccine: Serum Institute. For cervical cancer prevention. India's government providing free HPV vaccination for girls 9–14 years (Union Budget 2023).
Long COVID & immune dysregulation: Research shows persistent immune activation, autoantibodies, and T cell exhaustion in Long COVID patients — major area of immunological research 2023–2025.
| Topic | Key Fact | UPSC Relevance |
|---|---|---|
| Autoimmune diseases | Immune system attacks own cells. Examples: Rheumatoid arthritis (joints), Type 1 Diabetes (pancreatic β-cells), SLE (Lupus), Multiple Sclerosis (nerves), Psoriasis (skin). Failure of Suppressor T cells (Tregs). | Asked in science questions; related to organ transplantation rejection (CMI). |
| Allergies | Exaggerated immune response to harmless antigens (allergens). IgE binds to mast cells → histamine release → inflammation, hay fever, asthma, anaphylaxis. Treatment: antihistamines, immunotherapy. | IgE role in allergies is high-yield. Anaphylaxis = life-threatening allergic reaction. |
| HIV/AIDS & Immunity | HIV attacks CD4+ Helper T cells → immune collapse. CD4 count <200/μL = AIDS. Opportunistic infections then kill the patient. AIDS = Acquired Immunodeficiency Syndrome — immune system destroyed. | Classic UPSC question: HIV attacks CD4+ T cells. India has 3rd largest HIV burden globally. |
| CAR-T cell therapy | Patient's T cells engineered to have Chimeric Antigen Receptors (CARs) that recognise cancer cells → infused back into patient → cancer killer cells. First gene-edited therapy. India approved 2024. | Cutting-edge immunotherapy; likely UPSC CA question. |
| Organ transplantation | Cell-mediated immunity causes graft rejection. Host's cytotoxic T cells attack transplanted organ. Immunosuppressants (cyclosporine, tacrolimus) needed to prevent rejection. MHC matching reduces rejection risk. | Transplantation biology — CMI's role in rejection. |
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Practice MCQs — Immunity & Immune System
UPSC-style · Click an option to reveal answer
🛡️ Click any option to check your answer
Q1. Where do T lymphocytes MATURE? (Not where they originate.)
- (a) Bone marrow
- (b) Thymus
- (c) Spleen
- (d) Lymph nodes
T lymphocyte PRECURSORS are produced in the bone marrow, but T lymphocytes MATURE in the Thymus. This is a critical distinction. The thymus is a primary lymphoid organ located above the heart. T cell precursors migrate from bone marrow to thymus, where they undergo selection and differentiation into CD4+ (Helper T) and CD8+ (Cytotoxic T) cells via a process shown in the DN1→DN4→DP stages. The thymus is largest in infancy and involutes (shrinks) after puberty — this is why elderly people have weaker cell-mediated immunity. Memory trick: T cell = Thymus; B cell = Bone marrow.
Q2. HIV (Human Immunodeficiency Virus) primarily destroys which type of immune cells, leading to AIDS?
- (a) B lymphocytes
- (b) Cytotoxic T cells (CD8+)
- (c) Helper T cells (CD4+)
- (d) Neutrophils
HIV specifically targets and destroys Helper T cells (CD4+ T cells). HIV's surface glycoprotein gp120 binds to CD4 receptors on Helper T cells. Progressive destruction of CD4+ T cells collapses the entire adaptive immune system, because Helper T cells are the "central coordinator" — without them, B cells cannot produce antibodies (no humoral immunity), cytotoxic T cells cannot be activated (no CMI), and macrophages cannot be properly activated. When CD4 count falls below 200/μL, the patient develops AIDS and becomes susceptible to opportunistic infections (like PCP pneumonia, CMV, tuberculosis) that a healthy immune system would easily control.
Q3. Interferons are produced by which cells and provide protection against what?
- (a) Virus-infected cells; they protect neighbouring uninfected cells from new viral infections
- (b) B cells; they neutralise bacteria in the bloodstream
- (c) Plasma cells; they are a type of antibody against viral antigens
- (d) Macrophages; they destroy intracellular bacteria
Interferons are proteins secreted by virus-infected cells. When a cell is infected by a virus, it releases interferons to "warn" and protect neighbouring cells. Interferons bind to surface receptors of nearby uninfected cells and signal them to: (1) produce antiviral proteins that block viral replication; (2) increase MHC class I expression so infected cells are better recognised by cytotoxic T cells; (3) activate NK cells. This is a cytokine barrier of innate immunity. Interferons are so named because they "interfere" with viral replication. They are part of innate (non-specific) immunity — they work against many different viruses, not just one specific virus.
Q4. A child is born to a mother who has immunity against measles. The child will have temporary immunity against measles due to which mechanism?
- (a) Active immunity developed by the child's own immune system
- (b) IgM antibodies transferred through the placenta
- (c) IgA antibodies transferred through the placenta
- (d) IgG antibodies transferred through the placenta — passive immunity
The newborn receives passive immunity through IgG antibodies transferred via the placenta. IgG is the ONLY immunoglobulin class that can cross the human placenta (via FcRn receptors on placental cells). This maternal IgG protects the newborn for the first 3–6 months of life until its own immune system matures. It is called PASSIVE immunity because: (1) the child's immune system did not produce these antibodies — they were made by the mother; (2) no memory cells are formed; (3) protection is temporary (as maternal IgG is gradually degraded). Additionally, secretory IgA in colostrum (first breast milk) provides mucosal passive immunity. IgM is a pentamer — too large to cross the placenta.
Q5. MALT (Mucosa-Associated Lymphoid Tissue) constitutes approximately what fraction of lymphoid tissue in the human body, and where is it found?
- (a) 10%; only in the intestinal lining
- (b) 25%; in the spleen and thymus
- (c) 50%; lining the digestive, urogenital, and respiratory tracts
- (d) 75%; distributed throughout all major organs
MALT (Mucosa-Associated Lymphoid Tissue) constitutes approximately 50% of all lymphoid tissue in the human body. It is located in the lining (mucosa) of the three major body tracts: (1) Digestive system — includes Peyer's patches in the ileum, and lymphoid tissue throughout the gut; (2) Urogenital system — protects the reproductive and urinary tracts; (3) Respiratory system — includes tonsils and adenoids (nasopharyngeal-associated lymphoid tissue, NALT). MALT is crucial because the mucous membranes are the most common entry points for pathogens. It produces secretory IgA (the most abundant Ig in mucosal secretions) which prevents pathogens from crossing the mucosal surface into the bloodstream.
Q6. Which immunoglobulin class is responsible for allergic reactions such as hay fever and asthma?
- (a) IgG
- (b) IgM
- (c) IgA
- (d) IgE
IgE is responsible for allergic (hypersensitivity) reactions. Mechanism: On first exposure to an allergen (e.g., pollen, dust mite proteins), B cells produce IgE antibodies specific to that allergen. IgE binds to mast cells and basophils (via Fc receptors). On second exposure to the same allergen, it cross-links IgE on mast cells → mast cells degranulate → release histamine, prostaglandins, leukotrienes → cause allergic symptoms: runny nose, watery eyes, bronchoconstriction (asthma), skin rash, itching. In severe cases → anaphylactic shock (life-threatening). IgE also provides protection against parasitic worms (helminths). Treatment: antihistamines block histamine receptors; epinephrine (adrenaline) for anaphylaxis.
⚡ Quick Revision — Immunity & Immune System
| Topic | Key Facts for UPSC |
|---|---|
| Definition | Ability to defend against pathogens. Immune system = lymphoid organs + immune cells (WBCs) + soluble molecules (antibodies, cytokines). |
| Primary lymphoid organs | Bone marrow (all blood cells + B cells mature here) + Thymus (T cells mature here). "B goes to Bone, T goes to Thymus." |
| Secondary lymphoid organs | Spleen (filters blood), Lymph nodes (trap antigens from lymph/tissue), Tonsils, Peyer's patches, MALT (50% of lymphoid tissue; mucosal surfaces). |
| T cells | Mature in thymus. TH (CD4+) = Helper T — activates macrophages, cytotoxic T cells, B cells = central coordinator. TC (CD8+) = Cytotoxic — kills virus-infected cells and cancer cells. TS = Suppressor/Regulatory Tregs. |
| B cells | Mature in bone marrow. → Plasma cells (produce antibodies) + Memory B cells. Humoral immune response. BCR = membrane-bound antibody (IgM/IgD). |
| Macrophages | Myeloid origin (Monocyte → Macrophage). Phagocytosis + APC (presents antigens to TH via MHC). Bridge between innate and adaptive immunity. NK cells = lymphocytes of innate; kill without antigen presentation. |
| Antibody structure | Y-shaped; 2 heavy + 2 light chains; disulphide bonds; variable domains (antigen-binding, unique) + constant domains (class-determining). Bivalent (monomer). 5 classes: IgG, IgA, IgM, IgE, IgD. |
| IgG | Most abundant (75% serum). Dominant in secondary response. Only Ig crossing placenta → foetal passive immunity. Also in breast milk. |
| IgM | Pentamer. Dominant in primary response. Best complement-fixer. ABO blood group antibodies. First BCR on naive B cells. |
| IgA | Secretory IgA in mucosal secretions. Most abundant Ig in colostrum/breast milk. Prevents antigen entry into blood. Mucosal immunity. |
| IgE | Allergic reactions (mast cells → histamine). Also against parasites. Lowest serum concentration. |
| Innate immunity | Non-specific, from birth, no memory. 4 barriers: Physical (skin, mucus), Physiological (saliva, acid, fever), Cellular (neutrophils, macrophages, NK cells), Cytokine (interferons — from virus-infected cells to protect uninfected cells). |
| Acquired immunity | Specific, has memory. Primary response (slow, low) → memory cells → Secondary response (fast, high). Humoral (B cells, antibodies) + Cell-mediated (T cells, CMI). |
| Active vs Passive | Active: body makes own antibodies (slow, long-lasting, has memory) — infection/vaccination. Passive: readymade antibodies given (fast, short-lived, no memory) — placental IgG, colostrum IgA, anti-serum. |
| Vaccination | Exploits immunological memory. Types: Live-attenuated (BCG, OPV, MMR), Killed (Covaxin), Subunit (Hep B, HPV), Toxoid (tetanus), mRNA (Pfizer — Nobel 2023), Viral vector (Covishield). India: UIP, Mission Indradhanush, Polio-free 2014. |
| Key CA | Nobel 2023: mRNA vaccines (Karikó & Weissman). Nobel 2018: immune checkpoint (cancer therapy). ZyCov-D (world's first DNA vaccine, India). iNCOVACC (world's first intranasal COVID vaccine, India). CERVAVAC (India's HPV vaccine). CAR-T cell therapy approved India 2024. HIV → destroys CD4+ TH cells → AIDS. |
🚨 5 UPSC TRAPS — Immunity & Immune System:
Trap 1 — "B cells mature in the thymus; T cells mature in bone marrow" → REVERSED! WRONG! It is exactly the opposite: T cells mature in Thymus; B cells mature in Bone marrow. Both originate (precursors formed) in bone marrow. But T cell precursors migrate to the thymus for maturation. Memory trick: B = Bone, T = Thymus. This is one of the most commonly reversed facts in exams.
Trap 2 — "IgA crosses the placenta to give passive immunity to foetus" → WRONG! Only IgG crosses the placenta. IgA (secretory) is found in colostrum/breast milk and gives mucosal passive immunity to the newborn AFTER birth — not before (via placenta). The distinction: placenta = IgG; colostrum = IgA. IgM is too large to cross. IgE and IgD don't cross either.
Trap 3 — "Interferons are antibodies produced by plasma cells against viruses" → WRONG! Interferons are proteins (not antibodies) secreted by VIRUS-INFECTED CELLS — not by plasma cells. They are part of the INNATE immune system (non-specific, no memory). They warn and protect neighbouring uninfected cells. Antibodies are produced by plasma B cells and are specific to one antigen. Interferons act broadly against many viruses.
Trap 4 — "The primary immune response is stronger and faster than the secondary response" → WRONG! It is exactly opposite: Primary response is SLOW and WEAK (first exposure); Secondary response is FAST and STRONG (re-exposure). The secondary response is stronger because memory B and T cells are already present and recognise the antigen immediately. This is why you need multiple doses of some vaccines (primary course) and why booster doses dramatically amplify immunity.
Trap 5 — "Helper T cells directly kill virus-infected cells" → WRONG! Helper T cells (CD4+) do NOT directly kill infected cells. Their role is to COORDINATE/ACTIVATE other immune cells: they activate macrophages (for phagocytosis), activate Cytotoxic T cells (CD8+ — which actually kill infected cells), and activate B cells (for antibody production). It is the Cytotoxic T cells (CD8+) that directly kill virus-infected cells and cancer cells. HIV's devastating effect is precisely because it destroys Helper T cells — without them, the entire coordination of adaptive immunity collapses.
Trap 1 — "B cells mature in the thymus; T cells mature in bone marrow" → REVERSED! WRONG! It is exactly the opposite: T cells mature in Thymus; B cells mature in Bone marrow. Both originate (precursors formed) in bone marrow. But T cell precursors migrate to the thymus for maturation. Memory trick: B = Bone, T = Thymus. This is one of the most commonly reversed facts in exams.
Trap 2 — "IgA crosses the placenta to give passive immunity to foetus" → WRONG! Only IgG crosses the placenta. IgA (secretory) is found in colostrum/breast milk and gives mucosal passive immunity to the newborn AFTER birth — not before (via placenta). The distinction: placenta = IgG; colostrum = IgA. IgM is too large to cross. IgE and IgD don't cross either.
Trap 3 — "Interferons are antibodies produced by plasma cells against viruses" → WRONG! Interferons are proteins (not antibodies) secreted by VIRUS-INFECTED CELLS — not by plasma cells. They are part of the INNATE immune system (non-specific, no memory). They warn and protect neighbouring uninfected cells. Antibodies are produced by plasma B cells and are specific to one antigen. Interferons act broadly against many viruses.
Trap 4 — "The primary immune response is stronger and faster than the secondary response" → WRONG! It is exactly opposite: Primary response is SLOW and WEAK (first exposure); Secondary response is FAST and STRONG (re-exposure). The secondary response is stronger because memory B and T cells are already present and recognise the antigen immediately. This is why you need multiple doses of some vaccines (primary course) and why booster doses dramatically amplify immunity.
Trap 5 — "Helper T cells directly kill virus-infected cells" → WRONG! Helper T cells (CD4+) do NOT directly kill infected cells. Their role is to COORDINATE/ACTIVATE other immune cells: they activate macrophages (for phagocytosis), activate Cytotoxic T cells (CD8+ — which actually kill infected cells), and activate B cells (for antibody production). It is the Cytotoxic T cells (CD8+) that directly kill virus-infected cells and cancer cells. HIV's devastating effect is precisely because it destroys Helper T cells — without them, the entire coordination of adaptive immunity collapses.
