Question 9

Classify the hypersensitivity reactions. Briefly describe the pathophysiological processes of each reaction. Give an example of each reaction.

College Answer

Mechanisms of Immunological Injury
Mechanism	Pathophysiology	Disease types
Type I Immediate hypersensitivity IgE mediated	basophil & mast cell degranulation histamine, SRSA, ECFA, NCF immediate weal & flare	anaphylaxis atopy
Type II cell cytotoxicity IgG, IgM mediated	direct phagocytosis or cell lysis activation of complement, classical tissue deposition of complement	blood transfusions Goodpasteur's syndrome autoimmune cytopaenias
Type III Immune complex IgG, IgM, IgA mediated	tissue deposition of Ag-Ab complexes accumulation of PMN's, macrophages & complement	SLE serum sickness necrotising vasculitis
Type IV Delayed hypersensitivity T-cell mediated	T-cell induced mononuclear cell accumulation release of lymphokines & monokines often with ranuloma formation	TB, sarcoid Wegener's granulomatosis granulomatous vasculitis

Key points:

1. "Anaphylaxis" may be:

true anaphylaxis: a symptom complex following exposure of a sensitised individual to an antigen, produced by a type I hypersensitivity reaction, associated with IgE mediated mast cell degranulation
anaphylactoid reactions: indistinguishable from true anaphylaxis, however the immune nature of the reaction is either unknown, or not due to a type I hypersensitivity reaction, immediate generalised reaction a better term i End-organ effects, e.g. HI and H2receptors