Question
Active immunity.
Answer
Active immunity is the resistance to infection that develops when an individual’s own immune system is stimulated to produce antibodies and/or sensitized lymphocytes in response to an antigen — slow in onset (days to weeks) but long-lasting, since it involves formation of immunological memory.
Types:
- Natural active immunity — acquired following actual clinical or subclinical infection (e.g., immunity after recovering from measles or chickenpox).
- Artificial active immunity — acquired by deliberate immunization with a vaccine (live attenuated, killed/inactivated, toxoid, or subunit vaccine), stimulating the immune system without necessarily causing disease.
Key characteristics: slow onset (time needed for clonal expansion of B/T cells), but durable protection, often reinforced by booster doses; generates immunological memory (memory B and T cells), providing a faster, stronger secondary response on re-exposure — the basis of vaccination programmes and of the resurgence of a milder illness on natural re-exposure.
Mechanism: on first antigen exposure, naive lymphocytes with matching specificity are selected and undergo clonal expansion, differentiating into effector cells (plasma cells secreting antibody, or effector T cells) and long-lived memory cells; on re-exposure to the same antigen, the pre-existing pool of memory cells mounts a much faster and more vigorous secondary response.
Contrast with passive immunity: passive immunity (transfer of ready-made antibody — maternal antibody across the placenta, or immunoglobulin/antitoxin injection) acts immediately but is short-lived, since no memory is generated and the transferred antibody is gradually catabolised — active immunity is the basis of durable, long-term protection, which is the goal of vaccination programmes.