Cutaneous allergic vasculitis marked by antibody-antigen reaction at vessel wall; causes include microbes, drugs, toxic materials, and neoplasms. Acute form is marked by purpuric, hemorrhagic skin lesions on arms, legs, and buttocks; subacute form is marked by papules, urticaria, and necrotic lesions coalescing into plaques; and chronic form is marked by urticaria, petechiae, and maculopapular lesions.