Human hypersensitivity angiitis.

Human hypersensitivity angiitis is a disease involving deposition of immune complexes in which patients present with palpable purpuric lesions most often on the lower extremities and sometimes associated with multiple organ involvement. There are numerous potential antigens in the immune complex and the disease may be derived from an infectious organism such as Streptococcus or the hepatitis virus, from a drug, or from a variety of other chemicals which may be ingested or inhaled. In addition, the antigen may be part of another systemic disease. This complex of circulating antigen and antibody may become fixed within the vessel wall activating the complement cascade with elaboration of factors chemotactic for neutrophils. These cells then release lysosomal enzymes which result in destruction of the vessel wall and leakage of red blood cells into the tissue. This produces the typical purpura and palpability seen clinically. A central question in the pathogenesis of this disease is to explain why the immune complex is seemingly so selective in its site of deposition as it may be deposited in postcapillary venules in some patients and medium-sized muscular arteries in other patients. Part of the reason for this must be related to the lattice formation of a particular complex, while other reasons are likely related to alterations in vascular permeability, to the integrity of clearance mechanisms for the complex, or possibly even to a genetically determined defect of the phagocytic system.