a form of immediate hypersensitivity resulting in erythema, edema, hemorrhage, and necrosis observed in rabbits after injection of antigen to which the animal has already been sensitized and has specific IgG antibodies. The reaction is caused by the inflammation that results from the deposition of antigen-antibody complexes in tissue spaces and in blood vessel walls that activate complement, most of the damage seemingly being due to the polymorphonuclear leukocytes that phagocytize the deposits and release lysosomal enzymes. The phenomenon, described by Arthus, was in rabbits, but similar reactions (Arthus-type reactions) are observed in guinea pigs, rats, and dogs, as well as in humans. See Also: Arthus reaction(2). Syn: Arthus reaction(1).