Human hypersensitivity angiitis is an immune complex disease in which patients present with palpable purpuric lesions of the skin and may often have multiple organ involvement. The antigen may be derived from an infectious organism such as the hepatitis virus, streptococcus, or a drug, and complexes with antibody. Under circumstances of vascular turbulence or vessel wall dilatation this complex may become fixed, activating the complement sequence with elaboration of chemotactic factors for neutrophils. These cells release lysosomal enzymes resulting in vessel wall destruction. Red blood cells leak into the tissue producing purpura and the inflammatory infiltrate accounts for the palpability. Although many patients have skin lesions only, others may have involvement of joints, gastrointestinal tract, kidneys, and even the lungs. The central question in the pathogenesis of this disease is why the immune complex is so selective in its site of deposition. Part of the reason must be related to the lattice formation of a particular complex, while other reasons are related to host factors of altered vascular permeability, integrity of clearance mechanisms or even a genetically determined defect of the phagocytic system.