Immunology: Host Defence, Antigens, Antibodies and Vaccines
Domain 7 tests innate and adaptive immunity, antigen recognition, antibody structure and vaccine types. Questions often ask which immunoglobulin dominates a response, how vaccines differ, and why live vaccines are avoided in immunocompromised patients.
Core rules and concepts
- Innate immunity is immediate and non-specific: skin and mucous membranes, phagocytes, complement, inflammation, fever. Adaptive immunity is specific and has memory: B cells produce antibodies (humoral), T cells mediate cellular immunity.
- Antigens are substances recognised by adaptive receptors; epitopes are the actual binding sites. Haptens are small molecules that become immunogenic only when attached to a carrier.
- Antibody monomer: two heavy and two light chains, variable regions forming the antigen-binding site, constant regions determining class. IgG is the main blood antibody and crosses the placenta; IgM is a pentamer and appears first in infection; IgA is secretory and mucosal; IgE mediates allergy; IgD is a B-cell receptor.
- Vaccines: live attenuated (BCG, MMR), killed (IPV, pertussis), toxoid (diphtheria, tetanus), subunit/conjugate (Hib, some hepatitis B). Passive immunization gives ready-made antibodies.
Worked example
A child receives a first dose of a vaccine and later a booster. Reasoning: the first exposure produces a primary response with a lag, mostly IgM, and memory cells. The booster triggers a secondary response that is faster, larger and dominated by IgG. This is why booster doses are scheduled.
Mark-losing traps
- IgM is not the memory antibody; IgG dominates secondary responses.
- IgA is not the main placental antibody; IgG is.
- Live vaccines are generally contraindicated in severe immunodeficiency and pregnancy.
- Antigen and antibody are not the same; an antigen induces a response, an antibody is the product.