Histamine is a protein, acting as a vasodilator (widening of blood vessels) in inflammatory and allergic reactions and also increases the permeability of small vessels. It is secreted from the mast cells, which widely occur in the areolar connective tissue. Two other active substances secreted by mast cells are heparin, a proteoglycan, which prevents coagulation of blood vessels and serotonin, a protein, which acts as vasoconstrictor to stop bleeding and to increase blood pressure.