allergy
(al'ĕr-jē 
)
[ allo- + Gr. -ergia, work, activity]
An immune response to a foreign antigen that results in inflammation and organ dysfunction. Allergies range from annoying to life-threatening. They include systemic anaphylaxis, urticaria, eczematous dermatitis, hay fever, and rhinitis. They affect about 20% of Americans and can be triggered by inhalation (pollen, dust mites), direct contact (poison ivy), ingestion (drugs, foods), or injection (stinging insects, drugs). Allergic responses may be initiated and sustained by occupational exposure to allergens, and by foods, animals, fungal spores, metals, and rubber products. The most severe cases are often associated with Hymenoptera stings, penicillin products, radiological contrast media, and latex. SYN: hypersensitivity reaction.
SEE: allergen; atopy.
Etiology: The immune system has two main functions: first, to identify germs and parasites that may harm the body; and second, to use toxic defenses against attacks by these organisms. Allergic reactions occur when immune functions are turned on by an agent richly endowed with alien antigens. Once the immune system has been sensitized, subsequent exposure results in the binding of specific immunoglobulins (esp. IgE) or the activation of immunologically active cells, e.g., mast cells, basophils, or T cells. These can release inflammatory chemicals (histamines, kinins, interleukins) that create allergic symptoms.
Symptoms: Nasal inflammation, mucus production, watery eyes, itching, rashes, tissue swelling, bronchospasm, stridor, and shock are all symptoms of allergy.
Diagnosis: A history of exposure and reaction is crucial to the diagnosis of allergy. Tests for specific allergies include skin prick tests, intradermal injections, or blood tests (measurements of antigen-specific immunoglobulins).
Treatment: Avoiding allergens is the first step in treatment. Effective drugs for allergic symptoms include antihistamines, corticosteroids, and epinephrine, depending on the severity of the reaction. Antigen desensitization (immunotherapy) may be used by experienced professionals, but this technique may occasionally trigger severe systemic reactions.
Patient Care: Before any drug is given, the health care provider should determine if the patient has a history of allergy. Patients receiving injected drugs or blood products are closely observed for rash, itch, wheezing, or hypotension. If an allergic reaction begins, medications are prescribed for immediate management. Patients are taught to identify and avoid common allergens and to recognize an allergic reaction. The use of drugs for the chronic management of allergies is explained, and the patient is advised about potential adverse effects. If a patient needs injectable epinephrine for emergency outpatient treatment of anaphylaxis, both the patient and family are instructed in its use.
contact a.A type IV hypersensitivity reaction following direct contact with an allergen, usually affecting the skin.
SEE: contact dermatitis.
drug a.A type I, IgE-mediated hypersensitivity reaction to an administered drug, e.g., penicillin.
food a.An immunologic reaction to a food to which a patient has become sensitized. Sensitivity to almost any food may develop, but the most common food allergies are to milk, eggs, peanuts, wheat, shellfish, and chocolate. Because food allergies are type I reactions, symptoms can appear within minutes. Mild symptoms (urticaria, abdominal cramps, and gastrointestinal upset) are most common, but food allergies can also cause systemic anaphylaxis and vasovagal syncope.
Food allergies are identified by eliminating any foods suspected of causing symptoms and reintroducing them one at a time. Blood tests for IgE are useful in separating food allergies from abnormal metabolic or digestive responses to food. Desensitization to food allergies is impossible, and use of antihistamines, epinephrine, and corticosteroids cannot be used for prophylaxis. Many adverse reactions to foods are not allergic in nature but may be caused by toxic, metabolic, or pharmacological reactions.
SEE: anaphylaxis; desensitization.
human seminal plasma a.Seminal plasma allergy.
latex a.An immune reaction resulting from contact with products derived from the rubber tree, Hevea brasiliensis, or the chemicals added to latex in manufacturing. Latex antigens can be inhaled or absorbed through the skin. The allergic reaction may be mild (rashes, reddened skin) or severe (bronchospasm, anaphylaxis). In health care workplaces, where wearing latex gloves is common, nonlatex products have been substituted to reduce exposure. A nonallergic contact dermatitis caused by the powder used in latex gloves may be mistaken for a true latex allergy and is much more common.
outgrown a.An allergy, especially a food allergy, that fades over time or becomes clinically insignificant.
peanut a.An IgE-mediated immediate hypersensitivity reaction to the consumption of peanuts (the seeds of Arachis hypogaea). Peanut allergens are designated Ara by the World Health Organization.
Peanut allergy is the most important food allergy in the U.S., affecting more than a million people.
Reactions range from mild (rashes) to life-threatening (closure of the airway, cardiac dysrhythmias, coma).
Dietary exposure to peanuts in infancy may limit the likelihood of developing peanut allergy in childhood.
About 50 people die of peanut allergy in the U.S. each year. Most affected people remain sensitive throughout their lives; approx. 20% of children affected by peanut allergy become tolerant to peanuts over time.
People with known allergies to peanuts must avoid eating raw or processed peanuts and products containing or prepared with peanut oil. Those affected by peanut allergy should learn to watch for the signs of anaphylaxis (hives, pruritus, rashes in the skin creases, shortness of breath, choking, wheezing, stridor). People with known anaphylaxis to peanuts should carry epinephrine injectors and use them at the onset of a hypersensitivity reaction. (Repeated use may be necessary in persistent reactions.) Cross-reactivity to other legumes (peas, soy products) may affect some people and pose important health risks. Affected people should wear medical alert bracelets or necklaces identifying their condition. Densensitization can be accomplished with modified peanut allergens.
penicillin a.A hypersensitivity reaction to penicillin.
The condition is present in about 0.5% to 8% of the population.
Although different types of hypersensitivity reactions may occur, the most common and potentially dangerous are the type I (immediate) reactions mediated by immunoglobulin E.
Common symptoms include itching and swelling of the skin and mucous membranes, or, in cases of systemic anaphylaxis, cough, shortness of breath, wheezing, or cardiovascular collapse after taking penicillin.
A skin test can determine sensitivity to the drug but should only be done by an allergist in a controlled setting in which epinephrine is available as a rescue medication, and trained professional staff are prepared to respond to anaphylactic reactions.
No penicillin or other beta-lactam antibiotics (such as cephalosporins) should be given to penicillin-allergic patients. In those very rare situations in which an infection is susceptible only to penicillin and the infection is serious enough to risk the danger of anaphylaxis, the patient may be desensitized with gradually increasing doses of penicillin.
After the administration of penicillins, the patient should be monitored for anaphylaxis or adverse reactions such as hives or rashes. Anithistamines can be given orally or parenterally for rashes or hives; epinephrine is administered by injection for anaphylaxis.
seminal plasma a.A rare allergic reaction to contact with proteins in semen. It can produce irritation of the vulva and vagina in women or, in some instances, anaphylaxis. It is treated by avoiding exposure to seminal plasma, e.g., with condom use during intercourse, or with topical antiallergy agents such as cromolyn, applied before sexual intercourse. SYN: human seminal plasma allergy .