Immunology: Innate and Acquired Immunity
Introduction
This domain compares innate and acquired immunity, including barriers, phagocytes, complement, lymphocytes, antibodies, memory, and active versus passive protection. The exam tests the sequence of immune responses and the basis of vaccination.
Core rules
- Innate immunity is immediate, lacks antigen-specific memory, and uses physical barriers, phagocytes, natural killer cells, complement, and inflammation.
- Acquired immunity is specific and has memory; B cells produce antibodies, and T cells mediate helper and cytotoxic functions.
- Helper T cells recognise antigen presented with MHC class II; cytotoxic T cells recognise MHC class I.
- Active immunity develops after infection or vaccination and generates memory; passive immunity is transferred antibody or cells and is immediate but short-lived.
- Primary exposure produces mostly IgM, followed by IgG; re-exposure produces a faster, larger IgG response.
Worked example
A person receives a tetanus booster after a deep wound. The vaccine contains toxoid, so the immune system mounts an active response and memory B cells expand. If the patient also receives tetanus immunoglobulin, that provides passive antibody for immediate neutralisation. The immunoglobulin protects now but does not create memory, whereas the booster does.
Traps
Innate immunity is not random; it recognises conserved patterns. Passive immunity is not long-lasting and does not produce memory. IgM is not always the only first antibody, and a high total IgG does not prove protective immunity to a specific pathogen.