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Medical Definition of Type III hypersensitivity reaction
1. An immunologic category of diseases evoked by the deposition of antigen-antibody or antigen-antibody-complement complexes on cell surfaces, with subsequent involvement of breakdown products of complement, platelets, and polymorphonuclear leukocytes, and development of vasculitis; nephritis is common. Arthus phenomenon and serum sickness are classic examples, but many other disorders, including most of the connective tissue disease's, may belong in this immunologic category; immune complex disease's can also occur during a variety of disease's of known aetiology, such as subacute bacterial endocarditis. See: autoimmune disease. Synonym: immune complex disorder, type III hypersensitivity reaction. (05 Mar 2000)