Allergies

Alternative names:

allergic response

Definition:

A collection of symptoms caused by an immune response to substances that do not trigger an immune response in most people. See also food allergies; hay fever; allergy to mold, dander, dust; drug allergies; allergy testing; allergic reactions.

Causes, incidence, and risk factors:

Allergy is caused by hypersensitivity of the immune system leading to a misdirected immune response. The immune system normally protects the body against harmful substances such as bacteria, viruses, and toxins. Allergy occurs when the immune system reacts to substances (allergens) that are generally harmless and in most people do not cause an immune response.

The first exposure to the allergen causes a mild immune response that sensitizes the immune system to the substance (triggers the immune system to recognize the substance). The second and subsequent exposure to the allergen results in symptoms. The type of symptom that develops depends on the specific allergen, the part of the body where exposure occurs, and the way the immune system reacts to the allergen.

When an allergen enters the body of a person with a sensitized immune system, it triggers antibody production. Histamine and other chemicals are released by body tissues as part of the immune response. This causes itching, swelling of affected tissues, mucus production, muscle spasms, hives and rashes, and other symptoms. Symptoms vary in severity from person to person.

The part of the body contacted by the allergen will, in part, also affect symptoms. For example, allergens that are inhaled often cause nasal congestion, itchy nose/throat, mucus production, coughing, wheezing, or similar symptoms. Food allergies often include abdominal pain, cramping, or similar symptoms, although the whole body may be affected when the food is absorbed. Allergies to plants often cause skin rash. Drug allergies usually involve the whole body.

Many disorders are associated with, triggered, or worsened by allergies. These include hay fever, eczema, asthma, and many others.

Common allergens include environmental agents that contact the skin, breathing passages, or the surface of the eye (such as pollen; see also allergy to mold, dander, dust). Food allergies and drug allergies are common. Allergic reactions can be caused by insect bites, jewelry, cosmetics, and almost any substance that contacts the body.

Some people have allergic-type reactions to hot or cold temperatures, sunlight, or other physical stimuli. In some persons, friction (such as rubbing or vigorously stroking the skin) will cause symptoms. The mechanism that causes this is not well understood, but it is possible that minute changes in the chemistry of the skin may occur in response to physical stimuli and some component of this chemical change triggers the allergy. (See also atopic dermatitis and contact dermatitis.).

Allergies are common. Heredity, environmental conditions, number and type of exposures, emotional factors (stress and emotional upset can increase the sensitivity of the immune system), and many other factors can indicate a predisposition to allergies.