Immunology and Immunopathology
Innate and adaptive immunity
Know the components of innate immunity (barriers, phagocytes, complement, natural killer cells, pattern-recognition receptors such as Toll-like receptors) and adaptive immunity (T and B lymphocytes, antigen presentation via MHC I to CD8 cytotoxic cells and MHC II to CD4 helper cells). Understand immunoglobulin classes: IgM is the first responder and a potent complement activator; IgG crosses the placenta and dominates secondary responses; IgA is mucosal; IgE mediates type I hypersensitivity; IgD is a B-cell receptor component.
Hypersensitivity and autoimmunity
Classify the four types. Type I is IgE-mediated mast cell degranulation (anaphylaxis, atopy). Type II is antibody against fixed antigen (haemolytic disease, Goodpasture, myasthenia gravis). Type III is immune complex deposition (SLE, post-streptococcal glomerulonephritis, serum sickness). Type IV is T-cell mediated (contact dermatitis, tuberculosis granuloma, transplant rejection). Autoimmunity arises from loss of tolerance; know HLA associations such as HLA-B27 with ankylosing spondylitis and HLA-DR4 with rheumatoid arthritis.
Immunodeficiency and transplantation
Primary immunodeficiencies include X-linked agammaglobulinaemia (Bruton), DiGeorge syndrome (T-cell defect, thymic hypoplasia), and severe combined immunodeficiency. Secondary causes include HIV, malnutrition, and immunosuppression. Transplant rejection may be hyperacute (preformed antibodies), acute cellular or antibody-mediated, or chronic. Graft-versus-host disease occurs when donor T cells attack host tissue.
Traps
Do not confuse type II and type III: type II involves antibody bound to a fixed tissue antigen; type III involves circulating immune complexes. Complement deficiency of C3 increases susceptibility to encapsulated organisms; terminal complement deficiency predisposes to Neisseria.
Rule of thumb
If the disease improves with plasma exchange or is complement-mediated, consider type II; if it follows infection and deposits granular immune complexes, consider type III.