A structure or site, found on the surface of a cell or within a cell, that can bind to a hormone, antigen, or other chemical substance and thereby begin a change in the cell. For example, when a mast cell within the body encounters an allergen, specialized receptors on the mast cell bind to the allergen, resulting in the release of histamine by the mast cell. The histamine then binds to histamine receptors in other cells of the body, which initiate the response known as inflammation as well as other responses. In this way, the symptoms of an allergic reaction are produced. Antihistamine drugs work by preventing the binding of histamine to histamine receptors.