Anaphylactoid purpura: A form of blood vessel inflammation that affects small capillaries in the skin and the kidneys. It results in skin rash associated with joint inflammation (arthritis) and cramping pain in the abdomen. Anaphylactoid purpura frequently follows a bacterial or viral infection of the throat or breathing passages, and it is an unusual reaction of the body's immune system to this infection. It occurs most commonly in children. Generally a mild illness that resolves spontaneously, anaphylactoid purpura can sometimes cause serious problems in the kidneys and bowels. Treatment is directed toward the most significant area of involvement. Also known as Henoch-Schonlein purpura (HSP).