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
Anaphylaxis
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
Anaphylactoid
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