L-histidine is a protein amino acid that is found in the proteins of all life forms. Although most L-histidine is found in proteins, a small amount of free L-histidine does exist in plants and fermented foods. The naturally occurring dipeptides found in muscle, carnosine and anserine are both comprised of L-histidine and beta-alanine. L-Histidine is an essential amino acid that is significant in the growth and repair of tissues. It is important for the maintenance of the myelin sheaths which protect the nerve cells. It is also needed for the production of both red and white blood cells. Histidine also protects the body from radiation damage, helps to lower blood pressure, and aids in removing heavy metals from the system. Histamine, an important immune system chemical, is derived from histidine.

L-histidine is one of the 10 essential amino acids for infants. It has never been clear if L-histidine is an essential amino acid for adults. At the very least, it is a conditional essential amino acid for adults. That is, even though L-histidine is synthesized in adult human tissues, sufficient quantities may not be made to meet the physiological requirements imposed by certain stress or disease situations.

L-histidine is a solid water-soluble substance. Chemically, it is called (S)-alpha-amino-1H-imidazole-4-propanoic acid; alpha-amino-4 (or 5)-imidazolepropionic acid; L-2-amino-3- (1H-imidazol-4yl) propionic acid, and glyoxaline-5-alanine. Its IUPAC abbreviation is His, and its one-letter abbreviation is H.