mast cell A connective tissue cell which, on becoming injured at a site of tissue damage, releases certain chemicals. Some of these cause a restriction of lymph vessels, thus allowing localized swelling to occur. Another, called histamine, increases capillary permeability, allowing leucocytes to leave the circulatory system more readily and to congregate at the site of injury.

mast cell (mahst) n. a large cell in connective tissue. Mast cells contain heparin, histamine, and serotonin, which are released during inflammation and allergic responses.