Immunoglobulin E (IgE) functions as part of the body's immune system (its defence against 'intruders'), and is a protein associated with allergic reactions. IgE is normally found in very small amounts in the blood. Sensitization to such allergies happens when someone with a genetic predisposition to allergies is exposed to a potential allergen such as grass pollen or cat fur for the first time. If the person is exposed to the allergen again, these attached IgE antibodies recognise the allergen and cause the mast cells and basophils to release histamine and other chemicals, resulting in an allergic reaction that begins at the exposure site. The allergen-specific IgE antibody test is used to screen for an allergy to a specific allergen. It measures the amount of that suspected IgE antibody in the blood.