Mast cells of connective tissue contain:

Mast cells secrete heparin (and coagulant), histamine (vasodilator) and serotonin (vasoconstrictor) along with other enzymes and hormones such as cytokines, peptidoglycans, protease etc. relaxin is secreted by placenta. Calcitonin is secreted by thyroid gland. Melanin is secreted by dermal cells. Related Theory Mast cells contribute to homeostasis in the immune system. They serve as a first line of defense against antigens entering the body due to their location in the skin and mucosa.