Allergy

An allergy is what happens when the immune system responds to a substance that is, for most people, entirely harmless, as though it were a dangerous invader. Such trigger substances are called allergens, and common examples include house dust, dust mites, pollen grains from certain plants, animal dander (skin flakes and fur), certain foods (such as peanuts or shellfish), and some medicines.

The mechanism behind an allergic reaction centres on a specific class of antibody, IgE (introduced in section 7.1). On first exposure to an allergen, the immune system of a susceptible (allergic) person produces IgE antibodies specific to that allergen; these IgE molecules then attach themselves to the surface of specialised cells called mast cells, found throughout body tissues, particularly near the skin, airways, and gut. On a later exposure to the same allergen, the allergen binds to the IgE already sitting on the mast cells, triggering those cells to rapidly release chemical mediators — most importantly histamine — into the surrounding tissue.

Histamine and the other released chemicals are responsible for the familiar symptoms of an allergic reaction: sneezing, a runny or blocked nose, watery and itchy eyes, skin rashes or hives, and, in the airways, the narrowing that causes wheezing and breathlessness seen in allergic asthma. In rare but serious cases, a severe, whole-body allergic reaction called anaphylaxis can occur and requires emergency medical treatment. …