Paper I
Question
Compare and contrast active and passive immunity.
Answer
| Feature | Active immunity | Passive immunity |
|---|---|---|
| Source of antibody | Produced by the individual’s own immune system | Pre-formed antibody transferred from an outside source |
| Onset | Slow (days to weeks — time needed for clonal expansion) | Immediate |
| Duration | Long-lasting (months to years, often lifelong with booster) | Short-lived (weeks, as transferred antibody is catabolised) |
| Immunological memory | Generated (memory B/T cells) | Not generated |
| Response on re-exposure | Faster, stronger secondary response | No improvement — same limited protection |
| Natural example | Immunity following actual infection (e.g., recovering from measles) | Maternal IgG across placenta; IgA via colostrum |
| Artificial example | Vaccination (live attenuated, killed, toxoid, subunit) | Immunoglobulin/antitoxin injection (e.g., HTIG, rabies immunoglobulin) |
| Use | Long-term prevention (vaccination programmes) | Immediate protection after high-risk exposure, or in immunodeficient patients |
| Antigen requirement | Requires antigen exposure to stimulate response | No antigen exposure needed — antibody given directly |
Both forms of immunity are complementary: active immunity is preferred for durable, population-level prevention, while passive immunity provides a valuable bridge for immediate protection when there is insufficient time for active immunization to take effect (e.g., post-exposure prophylaxis) or when the individual cannot mount an adequate active response.