Mast cells (mastocytes or histominocytes) are widely occurring in areolar and other connective tissues. Occasionally, these cells from short pseudopodia and slowly wander about in the body. These secrete three active substances-heparin, histamine, serotonin. Heparin, a proteoglycan, is an anticoagulant. It prevents coagulation of blood in blood vessels. Histamine is a protein, acting as a vasodilator in inflammatory and allergic reactions. Serotonin is also a protein, acts as a vasoconstrictor to arrest bleeding and to increase blood pressure.