A protein molecule (most commonly on the surface of a cell) which reacts to a substance which stimulates it. This leads to further biochemical changes within the cell itself.

Cell-Surface Receptors Proteins in, on, or traversing the cell membrane that recognize and bind to specific molecules in the surrounding fluid. The binding may serve to transport molecules into the cell's interior or to signal the cell to respond in some way. Examples of such receptors include the alpha and beta receptors on blood vessels; the beta-1 receptor of the heart; the histamine receptor on mast cells and cytokine specific receptors. See also : T-cell receptors , Immunoglobulin D .