Immunity — Innate, Adaptive, Antibodies, and Vaccines

• Difference between innate (non-specific) and adaptive (specific) immunity
• How B cells produce antibodies (humoral immunity) and T cells kill infected cells (cell-mediated immunity)
• Structure and classes of antibodies (IgG, IgM, IgA, IgE, IgD)
• MHC I vs MHC II — what they present, to whom, and on which cells
• How vaccines exploit immunological memory
• Complement system, interferons, and monoclonal antibodies (hybridoma technology)

Level 1 — Foundations

Innate Immunity (Non-specific, present from birth)

• Physical barriers: skin (keratinized), mucous membranes, cilia (sweep pathogens out)
• Chemical barriers: lysozyme in tears/saliva (cleaves bacterial peptidoglycan), HCl in stomach (pH 2), defensins in skin
• Cellular: neutrophils (first responders, phagocytosis), macrophages (phagocytosis + antigen presentation), NK cells (kill virus-infected/tumor cells without MHC recognition), mast cells (release histamine in inflammation)
• Inflammatory response: vasodilation, increased vascular permeability, recruitment of neutrophils; cardinal signs — redness, heat, swelling, pain
• Fever: pyrogens (LPS, IL-1, IL-6, TNF-α) act on hypothalamus → prostaglandin E2 → reset thermostat

Adaptive Immunity (Specific, acquired)

• Requires exposure to antigen; has memory; takes 1–2 weeks on first exposure (primary response), days on re-exposure (secondary response)
• Two arms:
  • Humoral immunity: B lymphocytes → plasma cells → antibodies; effective against extracellular pathogens
  • Cell-mediated immunity (CMI): T lymphocytes; effective against intracellular pathogens, fungi, transplant rejection, tumors

B Cells and Antibodies

• B cells mature in bone marrow; each carries unique B-cell receptor (surface immunoglobulin)
• On antigen binding (+ T-helper cell signal) → clonal expansion → plasma cells (antibody factories) + memory B cells
• Antibody classes:
  • IgG: most abundant in serum, crosses placenta (passive immunity to fetus), opsonization, complement activation; 4 subclasses
  • IgM: pentamer, first antibody produced in primary response; most efficient at complement activation; ABO blood group antibodies
  • IgA: dimer in secretions (saliva, breast milk, tears, gut lumen) as secretory IgA (sIgA); first line in mucosal immunity
  • IgE: lowest concentration; binds mast cells and basophils via Fc receptor → degranulation with allergen cross-linking → allergy/anaphylaxis; also anti-parasitic
  • IgD: mainly on surface of naive B cells; function not fully understood; may play role in B-cell activation

T Cells

• Mature in thymus; CD4+ helper T cells and CD8+ cytotoxic T cells
• CD4+ T helper cells: recognize antigen on MHC II; help B cells (Th2) or activate macrophages (Th1); secrete cytokines (IL-2, IL-4, IL-5, IFN-γ)
• CD8+ cytotoxic T cells (CTL): recognize antigen on MHC I; kill infected/tumor cells by releasing perforin (pores) + granzymes (apoptosis) or Fas ligand → Fas pathway

Active vs Passive Immunity

Vaccines

• Exploit the secondary immune response (faster, stronger, due to memory cells)
• Types: live attenuated (MMR, OPV, BCG), inactivated (IPV, hepatitis A), subunit (hepatitis B — recombinant HBsAg), toxoid (tetanus, diphtheria — formaldehyde-inactivated toxins), mRNA (COVID-19 — Pfizer/Moderna)
• Herd immunity threshold: depends on basic reproduction number R₀; for measles (R₀ ~15), need ~95% coverage

Level 2 — JEE / NEET depth

MHC I vs MHC II

Clonal Selection Theory (Burnet, 1957)

• Each lymphocyte is pre-committed to one antigen specificity (clonally selected)
• Antigen selects and activates the matching clone → proliferation (clonal expansion)
• Most daughter cells = effector cells; some = long-lived memory cells
• Explains immunological memory and tolerance (self-reactive clones deleted in thymus/bone marrow = clonal deletion)

Antibody Structure

• Monomer: 2 heavy chains + 2 light chains (κ or λ) linked by disulfide bonds; Y-shaped
• Variable region (Fab — Fragment antigen-binding): N-terminal, hypervariable complementarity-determining regions (CDRs) contact antigen
• Constant region (Fc — Fragment crystallizable): C-terminal of heavy chains; binds Fc receptors on phagocytes (opsonization), activates complement (classical pathway — IgG and IgM), determines antibody class
• Valency: IgG monomer = 2 binding sites; IgM pentamer = 10 binding sites (but steric hindrance reduces effective valency)

Complement System

• Three pathways: classical (IgG/IgM-antigen complex), lectin (mannose-binding lectin), alternative (direct pathogen surfaces) — all converge at C3 convertase → C3b (opsonization) → C5 convertase → C5b → Membrane Attack Complex (MAC): C5b-6-7-8-9 → transmembrane pore → osmotic lysis of bacteria/enveloped viruses
• C3a and C5a: anaphylatoxins → mast cell degranulation → inflammation

Interferons

• Type I (IFN-α, IFN-β): produced by virus-infected cells; signal neighboring cells to upregulate antiviral proteins (2-5A synthetase/RNase L system degrades viral RNA; PKR phosphorylates eIF2α → halts translation); also upregulate MHC I
• Type II (IFN-γ): produced by Th1 and NK cells; activates macrophages (classical activation → enhanced killing via reactive oxygen/nitrogen species); upregulates MHC I and II; key in defense against intracellular bacteria (Mycobacterium, Listeria)
• Type III (IFN-λ): primarily at mucosal surfaces

Monoclonal Antibodies (Hybridoma Technology — Köhler and Milstein, 1975)

1. Immunize mouse with antigen → B cells from spleen
2. Fuse B cells with immortal myeloma cells (PEG or electrofusion) → hybridomas
3. Select in HAT medium (hypoxanthine-aminopterin-thymidine): unfused myeloma cells die (aminopterin blocks de novo purine synthesis; myeloma lacks HGPRT for salvage pathway); unfused B cells die naturally
4. Screen hybridoma clones for desired antibody by ELISA
5. Expand single clone → monoclonal antibody (mAb)

• Applications: Herceptin (trastuzumab — anti-HER2 for breast cancer), Rituximab (anti-CD20 for B-cell lymphoma), adalimumab (anti-TNF-α for rheumatoid arthritis), COVID-19 neutralizing antibodies, diagnostic pregnancy tests (hCG detection), ELISA

Colostrum and Passive Immunity

• Colostrum (first 2–3 days of lactation): rich in secretory IgA, IgG, lysozyme, lactoferrin, cytokines, growth factors
• sIgA in colostrum/breast milk: protects newborn gut mucosa from pathogens; not absorbed into blood (local mucosal immunity)
• Maternal IgG crosses placenta via FcRn (neonatal Fc receptor) from ~28 weeks gestation → systemic passive immunity in newborn (lasts ~6 months)

Trace the immune response when influenza virus enters the respiratory tract:

Step 1 — INNATE RESPONSE (0–4 hours)
Virus enters respiratory epithelium → epithelial cells detect via pattern
recognition receptors (PRRs): TLR3 (dsRNA), TLR7/8 (ssRNA), RIG-I (cytosolic)
→ NF-κB activation → production of Type I interferons (IFN-α/β) + TNF-α + IL-6

Step 2 — INNATE CELLULAR (4–96 hours)
Infected cells upregulate MHC I → NK cells survey and kill cells with
downregulated MHC I (missing-self recognition)
Alveolar macrophages phagocytose virus → produce IL-12 (activates NK cells),
IL-1β, IL-6, TNF-α → fever, acute phase response
Dendritic cells (DCs) phagocytose virus in tissue → migrate to draining lymph
node (mediastinal) → mature DC upregulates CCR7, MHC II, CD80, CD86 (co-stimulatory)

Step 3 — ADAPTIVE INITIATION (day 4–7)
In lymph node, mature DC presents viral peptide (e.g., hemagglutinin peptide)
on MHC II → activates CD4+ Th cell bearing matching TCR (+ CD28-CD80 co-stimulus)
→ CD4+ T cell proliferates → Th1 subset secretes IFN-γ, IL-2
Same DC presents endogenous viral peptide on MHC I → activates CD8+ CTL

Step 4 — HUMORAL ARM
Th2-polarized CD4+ cells help influenza-specific B cells (recognize native HA
surface protein) → B cell class switches (IgM → IgG) via IL-4/IL-13
→ Plasma cells produced → secrete anti-HA IgG antibodies
Antibodies: (a) neutralize virus (block HA binding to sialic acid receptors)
(b) opsonize virions for macrophage phagocytosis (c) activate complement → MAC

Step 5 — CELL-MEDIATED ARM
Activated CD8+ CTLs traffic to lung → recognize HA/NP peptide:MHC I on
infected cells → release perforin (pore) + granzyme B (activate caspases)
→ infected cell undergoes apoptosis → virus cannot replicate further

Step 6 — RESOLUTION AND MEMORY (week 2–3)
>90% effector cells die by apoptosis (contraction phase) — mediated by FasL, TNF,
withdrawal of survival signals (IL-2)
Surviving ~5–10% = memory T cells (CD45RO+, CD62L+, long-lived, self-renewing)
Long-lived plasma cells migrate to bone marrow → secrete IgG for years
Memory B cells recirculate; on re-exposure respond within 1–3 days (secondary
response: 10–100× more antibody, faster, predominantly IgG, higher affinity
due to affinity maturation in germinal centers)

• Draw and label the structure of an antibody molecule showing Fab, Fc, heavy chain, light chain, variable region, disulfide bonds
• Name the five classes of immunoglobulins and state one unique functional feature of each
• Distinguish between active and passive immunity with two examples of each
• Explain why the secondary immune response is faster and stronger than the primary response
• State the cell types involved in cell-mediated immunity and describe how a cytotoxic T cell kills a virus-infected cell
• Write the steps of hybridoma technology for monoclonal antibody production
• Why does IgM form a pentamer and what functional advantage does this give?
• Explain the role of interferons in antiviral defense

• Figure 8.1: Human body showing physical and physiological barriers (skin, mucus, cilia, stomach acid, lysozyme)
• Figure 8.2: Types of immunity (innate/acquired, active/passive) — flowchart
• Figure 8.3: Structure of an antibody (Fab, Fc, heavy/light chains, variable/constant regions)
• Figure 8.4: Primary and secondary immune response graph (IgM peak first, then IgG; secondary response higher and faster)
• Vaccination and immunization examples: BCG, OPV, DPT schedules from NCERT text

• Which immunoglobulin is found in colostrum as a dimer?
• Name two cells that express MHC Class II on their surface
• What is the role of the Fc region of an antibody?
• Which type of T cell is activated by MHC I-presented antigens?
• IgM is a ___-mer with a valency of ___
• What enzyme does lysozyme target in bacteria?
• Name the experiment that proved clonal selection theory was correct (hint: Burnet's theory supported by...)
• Stretch: Why are people with HIV/AIDS susceptible to opportunistic infections — connect to CD4+ T cell count and specific immune functions lost

NCERT Chapter 8 link: Human Health and Disease — Class 12 Biology Exam connections: NEET questions routinely ask antibody class properties, MHC I/II distinction, active vs passive immunity, and steps of hybridoma technology. 2–3 questions per year on average. Study strategy: Draw the antibody structure from memory daily for 5 days. Make a table of all 5 Ig classes with one unique fact each. Practice the primary/secondary response graph — draw it without looking.

• Use the platform-native live simulation or PhET-style tool for this topic (number line, Venn, physics playground, molecule builder, sensor dashboard, etc.).
• Mirror / body / home activity: physically do the concept (count objects, measure, role-play) and photograph or describe for portfolio.
• Voice or text reflection with AI Mentor: explain the concept to a younger student or family member.

• "Explain this concept to a Class 6 student using one real example from an Indian home, school, market, or festival."
• "What is one common mistake students make here, and how would you catch yourself making it?"
• Stretch: "How does this connect to coding, robotics, money, health, environment, or a future career?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• One hands-on project or measurement using the Drishti kit or household items that makes the concept physical.
• Direct link to at least one Future Skill track (Money Management, Green Tech, Cyber Defenders, Micro-Entrepreneurship, AI Mastery, Sustainable Living, Personality Development).
• Coding extension where relevant (simple script, simulation, or data logging).

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.