Histidine. Molecular model of the amino acid histidine. Its chemical formula is C6.H9.N3.O2. Atoms are represented as balls and are colour- coded: carbon (blue), hydrogen (gold), oxygen (red) and nitrogen (dark blue). Amino acids are the building blocks of proteins. Histidine is an essential amino acid, especially for children. It cannot be synthesised by the body and so needs to be obtained from food. It is found in most animal and plant proteins, being most concentrated in pork, poultry, cheese and wheat germ. Histidine is needed for growing new tissues and repairing damaged tissue. It also plays an important role in maintaining nerve cell functions and in producing red and white blood cells.