An allergy is a hypersensitivity disorder of the immune system. Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid. Allergy is one of four forms of hypersensitivity and is formally called type I (or immediate) hypersensitivity. Allergic reactions are distinctive because of excessive activation of certain white blood cells called mast cells and basophils by a type of antibody called Immunoglobulin E (IgE). This reaction results in an inflammatory response that can range from uncomfortable to dangerous.

Mild allergies like hay fever are very common in the human population and cause symptoms such as red eyes, itchiness, runny nose, eczema, and hives. Allergies can play a major role in conditions such as asthma. In some people, severe allergies to environmental or dietary allergens or to medication may result in life-threatening reactions called anaphylaxis. Food allergies and reactions to the venom of stinging insects such as wasps and bees are often associated with these severe reactions.

Risk factors for allergy can be placed in two general categories: namely, host and environmental factors. Host factors include heredity, gender, race, and age, with heredity being by far the most significant. However, there have been recent increases in the incidence of allergic disorders that cannot be explained by genetic factors alone. Four major environmental candidates are alterations in exposure to infectious diseases during early childhood, environmental pollution, allergen levels, and dietary changes.

Allergy Tests

A variety of tests exist to diagnose allergic conditions. Blood tests can be done to look for an allergen-specific IgE. Skin allergy testing is another method for medical diagnosis of allergies through the attempt to provoke a small, controlled, allergic response. A microscopic amount of an allergen (for example, tree pollen) is introduced to a patient's skin by means of either a prick test, which employs a needle or pin to break the skin's surface, or a patch test, where a patch containing the allergen is applied to the skin.

If an immuno-response is seen, it can be concluded that the patient has a hypersensitivity (or allergy) to that allergen. A response can take the the form of a rash (urticaria, also known as hives), or, worse—anaphylaxis (a serious reaction that can result in death). Further testing can then be performed in order to identify the specific allergen.

Allergy Treatments

Treatments for allergies include avoiding known allergens, use of medications such as antihistamines that specifically prevent allergic reactions, steroids that modify the immune system in general, and medications such as decongestants that reduce the symptoms. Many of these medications are taken by mouth; however, epinephrine, which is used to treat anaphylactic reactions, is injected. Immunotherapy employs the injection of allergens in order to gradually desensitize the body's response.

Allergy Testing

A patient receiving a skin allergy test.

This image is of a young man receiving an allergy prick test. A blond-haired woman in a white lab coat sits on the right of the image. On her left is a young brown-haired man. His arm is out, and the woman, presumably a doctor, is pricking his skin with a tool. On the table in front of them is a tray of vials.