Immune System

Question 1 (10 marks)

(a) $H_c = 1 - \frac{1}{15} = 1 - 0.0667 = 0.9333 = 93.3\%$

• Correct substitution (1); correct fraction 1/15 = 0.0667 (1); answer ≈ 93.3% (1). Why: below this fraction immune, each case still infects >1 person on average.

(b) Non-immune allowed = $(1 - 0.9333) \times 40000 = 0.0667 \times 40000 = 2667$ people (≈2666–2667).

• Fraction non-immune 0.0667 (1); ×40,000 (1); ≈2667 (1).

(c) For $R_0 = 2$: $H_c = 1 - 1/2 = 0.5 = 50\%$ (1). Only 50% must be immune vs 93.3% for measles (1). A lower threshold means fewer people need vaccination/prior infection to break transmission chains (1). Highly contagious pathogens (high $R_0$) require near-universal immunity (1).

Question 2 (12 marks)

(a) Day 28 (secondary) response to X: faster onset (1), much higher antibody magnitude/titre (1), predominantly IgG rather than initial IgM (1). Day 0 (primary) response: slower (lag phase), lower peak, IgM first then IgG (1). Clear comparison across all three criteria (1).

(b) Antigen Y is new, so no pre-existing memory cells specific to Y exist (1). Clonal selection: only lymphocytes with receptors matching an antigen are activated and proliferate (1). Y-specific naive cells must first be selected and expand, giving a slow primary-type response (1). X already has an expanded memory clone, hence rapid secondary response — but this is specific to X, not Y (1).

(c) Memory B cells (memory lymphocytes) (1) — long-lived, respond rapidly on re-exposure (1). Originate/mature in the bone marrow (B cells) (1). (Accept memory T cells maturing in thymus if candidate specifies T context.)

Question 3 (10 marks)

(a) MHC class I (1). All nucleated cells express MHC I (1). MHC I presents peptides from intracellular/endogenous proteins (e.g., viral proteins made inside the cell) (1); MHC II is used by professional APCs for extracellular/phagocytosed antigen, so is not appropriate for a virus replicating inside the cell (1).

(b) Cytotoxic T cells (CD8+ T cells) (1) recognise viral peptide on MHC I (1). They release perforin (pores) and granzymes (1) inducing apoptosis of the infected cell (1).

(c) Extracellular bacteria are engulfed by phagocytes/APCs and displayed on MHC class II (1) to activate helper (CD4+) T cells — an exogenous pathway (1).

Question 4 (10 marks)

(a) Patient A = allergy (1): rapid hypersensitivity to a harmless external antigen (pollen) (1). Patient B = autoimmune disorder (1): immune system attacks the body's own proteins/tissues (1).

(b) Antibody class IgE (1); mast cells (accept basophils) degranulate (1) releasing histamine (1). Effect: vasodilation / increased capillary permeability / bronchoconstriction / mucus secretion / itching (any one) (1).

(c) Self-tolerance = failure to distinguish self from non-self (1); self-reactive lymphocytes were not eliminated/deactivated, so they attack the body's own cartilage antigens (1).

Question 5 (8 marks)

(a) Snakebite antivenom = passive, artificial (1)(1) — antibodies made externally, injected. Breast milk = passive, natural (1)(1) — pre-made antibodies transferred naturally.

(b) The recipient's own lymphocytes are not activated / no antigen exposure to trigger clonal selection (1). No memory cells are formed (1). Received antibodies are broken down/degraded over weeks (1), giving only short-term protection (1).

[
  {"claim":"Measles herd immunity threshold with R0=15 is about 93.3%","code":"Hc = 1 - Rational(1,15); result = abs(float(Hc)-0.9333) < 0.001"},
  {"claim":"Non-immune allowed in 40000 town at R0=15 is about 2667","code":"n = (1 - (1 - Rational(1,15)))*40000; result = abs(float(n)-2666.67) < 1"},
  {"claim":"Herd immunity threshold for R0=2 is 50%","code":"Hc2 = 1 - Rational(1,2); result = float(Hc2)==0.5"}
]