A mast cell is a granulocyte (a type of white blood cell) found in connective tissue, for example around blood vessels or in the skin. Mast cells don't usually circulate in the blood itself. They release histamine and serotonin during an allergic response. This release is triggered by the binding of antigen to a type of antibody (immunoglobulin E, or Age) that is bound to the mast cell.
The release of histamine-containing granules by mast cells sets off a series of events that ultimately results in a swelling. Sometimes, however, the mast cells begin to grow out of control and form a skin tumor.