Anaphylaxis is a rare and severe allergic reaction that causes symptoms such as shock and bronchospasm. It may be a complication of immunization. It may also be an abnormal response to an antigen (a foreign substance, usually a protein) that does not bother most people but elicits symptoms in those who have an inherited hypersensitivity to it. Well-known antigens that can cause anaphylaxis include penicillin, insect venom, pollen extracts, fish, shellfish and nuts.
The cause of anaphylaxis was first explored in the 1920s when horse serum - being used to make antitoxin for diphtheria, scarlet fever, tetanus, and tuberculosis - was found to induce the reaction. Scientists have since discovered that the explosive reaction of anaphylaxis is the culmination of a complicated sequence of events.
Anaphylaxis usually begins a few minutes after injection of the offending substance and is usually evident within 15 minutes. Symptoms can include sneezing, coughing, itching, "pins and needles" sensation of the skin, flushing, facial oedema, urticaria, anxiety, respiratory difficulties and hypotension, which may progress to shock and collapse. Cardiovascular collapse can occur without respiratory symptoms. Roughly 60 percent to 80 percent of anaphylactic deaths are caused by an inability to breathe because swollen airway passages obstruct airflow to the lungs. The second most common cause of anaphylactic deaths--about 24 percent by one estimate--is shock, caused by insufficient blood circulating through the body.
Anaphylaxis must be distinguished from fainting (vasovagal syncope), which is a more common and benign reaction, of virtually immediate onset and simply managed by placing the patient in a recumbent position. With anaphylaxis, changes develop over several minutes, may involve multiple body systems (skin, respiration, circulation) and may progress to unconsciousness only as a late event in severe cases. Rarely is unconsciousness the sole manifestation of anaphylaxis.
Anaphylaxis is one of the rarer events reported in the post-marketing surveillance system for vaccine adverse events. Based on the five years of complete data between 1990 and 1995, in Canada the annual rate of anaphylaxis ranges from 0.11 to 0.31 reports per 100,000 doses of vaccines distributed.
However evidence compiled in 2001 indicates that up to 15% of those in the USA are at risk for this condition, which was once considered a rare event. Researchers say that anaphylaxis can be triggered by food, insect stings, drugs and latex. They found that nearly 10% of the known cases of anaphylaxis were triggered by penicillin, 0.5% to 5% by inset stings and 0.0004% by food. Based on the 1999 US population of 272 million, the population at risk for anaphylaxis from penicillin is nearly 27.2 million, from insect stings is 1.36 million to 13.6 million, while only 1,099 cases of anaphylaxis were caused by food allergy.