The mast cell, with its 500-1000 metachromatic granules in the cytoplasm, is found in practically all connective tissue structures of the body. It is preferentially located along blood and lymph vessels, especially in the area of the terminal circulation in loose connective tissue. In parenchymal organs, which contain little connective tissue, such as the liver, kidneys, and heart muscle, mast cells are found in the adventitia of the vessels and in the regional connective tissue. The number in these parenchymal organs is therefore relatively low. A large number of mast cells are found in serous membranes, such as the pleura, pericardium, peritoneum, as well as in the meninges, and particularly in the dermis of the skin, especially at the acral sites. Nearly 100% of the high tissue histamine levels are deposited in these cells. Tissue mast cells show a response to blood.