Question 9

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

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College Answer

Mechanisms of Immunological Injury



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