Generally, antigen-spesific IgE antibodies on mast cells, or basophils, interact with previously encountered antigens mediating anaphylaxis. Mast cells are found in large numbers beneath cutaneous and mucosal surfaces and are closely associated with blood vessels and peripheral nerves. Basophils, most closely related to eosinophils, function similarly to mast cells. Basophils are present in the circulation, while mast cells are present only in tissue but in much greater numbers. When either of these cells types is triggered, a biphasic release of mediators occurs. The pathophysiology of anaphylaxis is clinically defined by the physiological effects of the immediate-phase and late-phase mediators on the target organs. By definition, anaphylaxis involves the cardiovascular, respiratory, gastrointestinal, or epidermal system; in most cases, multiple organs are involved.