Allergen
An allergen is any substance (antigen), most often eaten or inhaled, that is recognized by the immune system and causes an allergic reaction. Dust, pollen and pet dander are all common allergens, but it is possible to be allergic to anything. Most humans mount significant IgE (immunoglobulin E (IgE) is a class of antibody) responses only as a defense against parasitic infections. However, some individuals mount an IgE response against common environmental antigens. This hereditory predisposition is called atopy. In atopic individuals, non-parasitic antigens stimulate inappropriate IgE production, leading to type I hypersensitivity. A nonparasitic antigen capable of stimulating a type I hypersensitivity reaction in atopic individuals is called an allergen. Sensitivities vary from one person to another and it is possible to be allergic to an extraordinary range of substances. Dust, pollen and pet dander are all common allergens, but it is possible to be allergic to anything from chlorine