Allergy

An allergy is a harmful immune response to something most people tolerate with no trouble, such as pollen, dust mites, animal dander, mold spores, certain foods, insect venom or a drug. The trigger is called an allergen. Most allergies are type I, or immediate, hypersensitivity reactions, and both inherited tendency and environment shape who develops them.

The process has two stages. At first contact, helper T cells push B cells to make IgE antibodies against the allergen, and these antibodies settle on mast cells in the skin, airways and gut. Nothing is felt yet, but the person is now sensitized. On a later exposure, the allergen links neighboring IgE molecules together, and within minutes the mast cells pour out histamine and other mediators. Blood vessels widen and leak, airway muscle tightens and mucus increases, giving sneezing, itching, hives, swelling or wheezing. A reaction across the whole body, anaphylaxis, can drop blood pressure and close the airway, and can kill.

Allergists identify triggers with skin prick tests, where a positive result is a small itchy bump ringed by redness, or with blood tests for allergen-specific IgE. Management combines avoiding the allergen, antihistamines and other drugs, epinephrine for anaphylaxis and, for some allergies, gradual desensitization known as allergen immunotherapy.