Cell releasing histamine. Computer illustration of a mast cell releasing histamine (blue bubbles) in a blood vessel. Mast cells are a type of leucocyte (white blood cell). They contain the chemical mediators histamine, serotonin and heparin. Histamine is released from mast cells in response to an allergen (not shown), causing a localized inflammatory immune response. When an allergen is encountered, B cells (not seen) produce antibodies which bind to protein receptor molecules on the surface of the mast cell. When two antibodies are cross-linked with an antigen the cell is activated to release its histamine by exocytosis. Histamine causes capillaries to dilate, smooth muscle to contract, and many other effects.