An<unk>  is a nonparasitic antigen capable of stimulating a type-I hypersensitivity reaction in atopic individuals.
Most humans mount significant IgE 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. 
Sensitivities vary from one person to another and it is possible to be allergic to an extraordinary range of substances.
Allergen
Inhalant
Tobacco
Phytochemical
Marine toxins
Food additive
Generic drug
Antigen