Sensitizing substances include serums (usually horse serum), vaccines, allergen extracts, enzymes (such as L-asparaginase), hormones, penicillin and other antibiotics, sulfonamides, local anesthetics, salicylates, polysaccharides, diagnostic chemicals (sulfobromophthalein, sodium dehydrocholate, and radiographic contrast media), foods (legumes, nuts, berries, seafood, and egg albumin) and sulfite-containing food additives, and insect venom (honeybees, wasps, hornets, yellow jackets, fire ants, mosquitoes, and certain spiders).

A common cause of anaphylaxis is penicillin, which induces anaphylaxis in 1 to 4 of every 10,000 patients treated with it. Penicillin is most likely to induce anaphylaxis after parenteral administration or prolonged therapy and in atopic patients who are allergic to other drugs or foods.

An anaphylactic reaction requires previous sensitization or exposure to the specific antigen, resulting in the production of specific immunoglobulin (Ig) E antibodies by plasma cells. This antibody production takes place in the lymph nodes and is enhanced by helper T cells. IgE antibodies then bind to membrane receptors on mast cells (found throughout connective tissue, often near small blood vessels) and basophils.

On reexposure, the antigen binds to adjacent IgE antibodies or cross-linked IgE receptors, activating a series of cellular reactions that trigger degranulation—the release of powerful preformed chemical mediators (such as histamine, prostaglandins, and platelet activating factor) from mast cell stores. IgG or IgM enters into the reaction and activates the release of complement fractions.

This acute phase of the response occurs within minutes of exposure. Because of the systemic nature of the exposure, activation of mast cells is widespread, and the massive release of these powerful mediators near blood vessels leads to vascular collapse by stimulating contraction of certain groups of smooth muscles and by increasing vascular permeability. In turn, increased vascular permeability leads to decreased peripheral resistance and plasma leakage from the circulation to extravascular tissues (which lowers blood volume, causing hypotension, hypovolemic shock, and cardiac dysfunction).

In the later phase of this response (8 to 12 hours later), other mediators are synthesized and released, including chemokines, leukotrienes, and cytokines. These agents mediate the inflammatory response by recruiting eosinophils and lymphocytes. This delayed response may be less dramatic than the acute phase of anaphylaxis, but with a diffuse inflammatory response, further smooth-muscle contraction and edema can occur and progress to grave systemic symptoms.

Cardiovascular signs include hypotension, shock, and sometimes cardiac arrhythmias, which, if untreated, may precipitate circulatory collapse.