Classification of Immune-Mediated Injury Hypersensitivity

I Clonal expansion of B Antigen binds to cell Anaphylactic response to 

IV Clonal expansion of T cells activated, release contact dermatitis 

Type III reactions are the result of antigen-antibody (IgG) complexes that accumulate in tissues or the circulation, activate macrophages and the complement system, and trigger the influx of granulocytes and lymphocytes (inflammation). This is sometimes referred to as the Arthrus reaction and includes postinfection sequelae such as rheumatic heart disease. Farmer s lung, a pneumonitis caused by molds has been attributed to both type III and type IV, and some of the late phase response (4-6 hours after exposure) in asthmatics may be the result of Arthrus-type reactions. 

The different types of immune-mediated injury are not mutually exclusive. More than one hypersensitivity mechanism may be involved in the response to a particular antigen. Also the resulting pathology, particularly that caused by type III and IV, reactions may appear very similar, although the mechanisms leading to the effect are different. 

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