Defence Against Disease · Paper 2 — Free Biology HL Practice Questions

A researcher studies the effectiveness of two experimental vaccines against a bacterial pathogen that produces a potent exotoxin. The vaccines are tested in three groups of ten mice each: - Group A: vaccinated with a chemically inactivated exotoxin (toxoid) at week 0 and week 4. - Group B: injected with a saline placebo at week 0 and week 4. - Group C: injected with a single dose of a live attenuated bacterial vaccine at week 0. At week 8, all mice are challenged with a lethal dose of the live pathogen. Results are shown below. Group — Survival — Time to symptom onset — Outcome A — 10/10 — 48 hours — Resolved by 72 hours B — 0/10 — 12 hours — Death by 24 hours C — 9/10 — 36 hours — Resolved by 60 hours; one mouse died at 30 hours

Severe combined immunodeficiency (SCID) is a genetic disorder in which both B-lymphocytes and T-lymphocytes are absent or non-functional. A patient with SCID received a bone marrow transplant from a healthy sibling. The table below shows lymphocyte counts in the patient's blood over 12 months following the transplant, alongside normal reference ranges. Time (months) — T-cells $(\text{cells}\,\mu\text{L}^{-1})$ — B-cells $(\text{cells}\,\mu\text{L}^{-1})$ 0 — 0 6 — 800 — 200 1200 — 600 Normal range — 800–2000 — 100–500