Immunology and Host Defence Mechanisms
Domain 6 tests the division of labour between innate and adaptive immunity, antibody structure and classes, and the basis of vaccination and hypersensitivity.
Innate immunity
First-line defences are skin, mucous membranes, cilia, secretions and normal flora. Second-line defences include phagocytes (neutrophils and macrophages), natural killer cells, complement, interferons, inflammation and fever. Innate immunity is rapid, non-specific and has no memory.
Adaptive immunity
Antigen-presenting cells present processed antigen with MHC to helper T cells. B cells become plasma cells that secrete antibodies, and some become memory cells. Cytotoxic T cells kill infected cells. Humoral immunity targets extracellular pathogens and toxins; cell-mediated immunity handles intracellular pathogens, viruses and cancer. An antibody has two heavy and two light chains, variable antigen-binding sites and a constant region that determines class. IgM appears first in the primary response; IgG is most abundant and crosses the placenta; IgA is secretory; IgE mediates allergy; IgD is a B-cell receptor. The secondary response is faster, larger and higher affinity.
Vaccines and hypersensitivity
Active immunity follows infection or vaccination and is long-lasting; passive immunity through maternal antibody or antiserum is immediate but short-lived. Vaccines include live attenuated, killed, toxoid, subunit, conjugate and mRNA types. Hypersensitivity types are I (IgE allergy), II (antibody-mediated), III (immune complex) and IV (delayed, T-cell mediated, as in the tuberculin reaction).
Worked example
A vaccinated child exposed to measles has mild or no disease. Reasoning: vaccination generated memory B and T cells; on re-exposure, the secondary response rises faster and neutralises the virus before extensive replication.
Traps
Innate immunity has no memory. Antibodies do not directly kill; they neutralise, opsonise and activate complement. Live vaccines are avoided in severe immunodeficiency. IgE is not the main antibacterial antibody; IgG is.