Anaphylaxis is a severe and rapidly developing allergic reaction that may involve eczema, swelling in the face or throat, drops in blood pressure, and digestive upset. Eczema is a skin condition in which patches of skin become rough, itchy, and inflamed. Anaphylaxis is mediated by immunoglobulin E (IgE). It presents almost immediately, with a severe reaction occurring within minutes of secondary exposure to the causative antigen. Because IgE is the mediator, anaphylaxis is an extreme type I hypersensitivity.
Symptoms of anaphylaxis are varied but tend to involve a combination of rash, drops in blood pressure (causing lightheadedness), digestive upset, and swelling in the throat and face. In more extreme reactions, bronchial contraction and fluid collection in the larynx result in respiratory distress, and reductions in blood pressure cause acute vascular collapse, or the collapse of blood vessels leading to oxygen deprivation in the body tissues. Without rapid treatment severe anaphylaxis can stop breathing and circulation, resulting in death.
Allergens causing anaphylaxis are environmental in origin but can come from a wide range of sources. Common antigens are from foods, drugs, insect venom, and latex. In some people, latex allergens bind with IgE antibodies on mast cells and basophils. This causes a release of histamine and other mediators that results in the symptoms of itching, sneezing, rash, and even anaphylaxis. Many foods result in anaphylactic allergic reactions, including nuts and peanuts, shellfish, and eggs. Similarly, common drugs may induce anaphylaxis, including beta-lactam antibiotics such as penicillin and nonsteroidal anti-inflammatory medications such as ibuprofen.
The main preventive action against anaphylactic reactions involves avoiding the causative allergen. This is reasonable for antigens such as those arising from a penicillin allergy, but avoidance is more difficult for allergens in foods and is not necessarily possible for allergens such as insect venom. Some patients susceptible to anaphylaxis may respond well to desensitization treatment in which small quantities of the antigen are injected and the quantity is increased over time. Desensitization is more effective against allergies for insect venoms than for food allergies, which can be quite dangerous.
Treatment of anaphylaxis with antihistamines tends to be ineffective because the histamines causing the reaction have already been released by the time the antihistamine can be administered. During an anaphylactic reaction, the causative antigen is bound by mast cells that release histamine. The histamines induce intense bronchial smooth muscle contraction, increased vascular permeability, and increased secretions (nasal, bronchial, and gastric). Fortunately, intramuscular injection of epinephrine provides an efficacious treatment. It acts by constricting the blood vessels, which helps decrease swelling and increase blood pressure. Epinephrine increases heart contraction and rate and relaxes muscles around the lung airways, which opens the airways and improves breathing. It is recommended that patients with a history of anaphylaxis constantly carry a self-injectable dose of epinephrine in the event that they come into contact with their causative allergen.