Mast cells (which secrete histamine, heparin, and other mediators) are found throughout the body in connective tissue, particularly in tissues adjacent to blood vessels and near body surfaces (skin, respiratory tract, digestive system). Areolar tissue is a specific type of loose connective tissue that is especially rich in mast cells. Histamine released from mast cells causes vasodilation and increased vascular permeability in allergic reactions and inflammation.