S1i: Explain the immunological basis of hypersensitivity

Hypersensitivity – refers to an abnormal reaction produced by a normal immune system

Type I  Anaphylactic – IgE antibodies on surface of mast cells bind to foreign Ag causing mass MC degranulation.  Releases into circulation:  histamines, serotonin, leukotrientes, platelet activating

factor and heparin.  Resulting in VD, increased cap perm, mucous secretion and sm m spasm.  <1hr following exposure.  Eg Suxamethonium Anaphylaxis

Type II  Antibody-dependant – IgE & IgM bind to Ag and activate MF, NK and classical complement pathway → results in target cell lysis.  Eg blood transfusion reactions

Type III  Immune-complex mediated – Ag-Ab complexes in plasma deposit in vessels and tissues and activate the classical complement pathway.  SLE

Type IV  Delayed – cell mediated, does not involve Ab/complement.  T cells previously sensitised to Ag become activated on re-exposure and damage the cell.  Eg contact dermatitis

Anaphylaxis v Anaphylactoid


Prior exposure

No reaction on first Ag exposure

IgE mediated

Severe reaction may be fatal

Reaction not related to dose of Ag

Eg Anaphylaxis to SUX


No prior exposure

Reaction can occur on first exposure

Not IgE mediated

Reaction less severe

Severity is related to dose of agent

Eg reaction to IV contrast