Pathogenic immune complexes formed between antigen and antibodies tend to occur during periods of antigen excess. When the immune complexes precipitate into the tissues, they fix complement, leading to an intense immune reaction. In immune complex vasculitis, immune complexes deposit in the vascular endothelium or in capillary beds, such as those found in the skin, kidneys, or lungs. The most common skin manifestation of small vessel vasculitis is palpable purpura. Hypersensitivity vasculitis is characterized by immune complex deposition in capillaries, postcapillary venules, and arterioles. The usual causes are either medications (e.g., penicillins, sulfonamides, or cephalosporins) or infections. Cryoglobulinemic vasculitis is usually caused by hepatitis C infections. The antigens involved in cryoglobulinemic vasculitis are portions of the hepatitis C virus (HCV) virion. The relevant antibodies are both IgG and IgM, leading to the designation mixed cryoglobulinemia. Henoch-Schönlein purpura is associated strongly with IgA deposition within blood vessel walls. Hypocomplementemic urticarial vasculitis has many features that overlap with systemic lupus erythematosus.
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