MI2.{4,6} | Immunity in Infection & Immunoprophylaxis — Summary & Reflection

REFLECT

Consider these integrative scenarios:

1. A young doctor is reviewing India's decision to switch from OPV to IPV as the primary polio vaccine. OPV generates mucosal IgA in the gut, which IPV does not. In what clinical and epidemiological situations would OPV's mucosal immunity be particularly advantageous? Why does IPV provide superior individual protection while OPV may offer better community-level transmission interruption?

1. A tribal village in Odisha has very low measles vaccination coverage. An NGO campaign achieves 60% vaccination coverage in one month. The district health officer says this is insufficient to prevent an outbreak. Based on herd immunity principles, explain why, and calculate the minimum coverage needed.

1. An HIV-positive patient with CD4 count 150 cells/μL requires rabies post-exposure prophylaxis after a dog bite. Should they receive active (vaccine) or passive (HRIG) immunisation, or both? Justify from immunological principles.

KEY TAKEAWAYS

Key take-home points from this module:

• Extracellular bacteria are controlled primarily by antibody (opsonisation, complement); intracellular bacteria require Th1-driven CMI and macrophage activation. TB granuloma = successful CMI containment.
• Viral infections are controlled first by Type I interferons and NK cells (innate), then by CD8+ CTLs and neutralising antibody (adaptive). ADE explains severe secondary dengue.
• Fungal infections require neutrophils (innate) and Th1/Th17 CMI (adaptive). Helminths drive Th2 responses → IgE and eosinophilia.
• Active immunisation generates memory (CMI + antibody); live vaccines give strongest, longest protection. Conjugate vaccines convert TI polysaccharide antigens to TD, enabling infant immunisation.
• Passive immunisation provides immediate antibody without memory; natural passive immunity from maternal IgG wanes by 4–6 months of age.
• Herd immunity threshold (HIT = 1 − 1/R₀) determines the vaccination coverage required to break transmission chains; measles requires >92% coverage.