Region of an allergen/antigen that reacts with the binding sites of antibodies or T-cell receptors. B lymphocytes secrete antibodies ( immunoglobulins) having binding sites against epitopes, whereas T lymphocytes have heterodimeric T cell receptors (ab, gd) having binding sites against epitopes.
IgE antibodies react preferentially with allergenic regions which are caused by their native spatial structure (conformation-dependent epitopes).
The T-cells only react with peptide fragments of the allergens, which must also be bound to the body's own MHC class II molecules.