Histidine is a colorless, soluble, crystalline essential amino acid, which is the precursor of histamine. Histidine (C₃H₃N₂CH₂CHNH₂COOH) is necessary for growth and repair of tissue, production of red and white blood cells (it is a component – 8.5 percent by weight – of hemoglobin), and preservation of myelin sheaths. Nutritional sources of histidine include dairy products and meat.