An allergen is usually an inert substance (e.g. pollen, house dust mite faeces) that in some individuals can trigger the generation of an (inappropriate) antigenic response. Mediated by TH2 lymphocytes, it causes B-Lymphocytes to produce lgE.
Subsequent exposure of a sensitized individual to the allergen is therefore able to cross-link IgE antibodies on the surface of mast cells and trigger an immune response and histamine release.