Histidine (abbreviated as His or H) is an alpha-amino acid. The L-isomer is one of the 22 proteinogenic amino acids, i.e., the building blocks of proteins. It is classified as a charged, polar because of the hydrophilic nature of the imidazole side chain.

L-Histidine (L-(-)-Histidine) is a semi-essential amino acid (children should obtain it from food) needed in humans for growth and tissue repair, L-Histidine is important for maintenance of myelin sheaths that protect nerve cells and is metabolized to the neurotransmitter histamine. Histamines play many roles in immunity, gastric secretion, and sexual functions. L-Histidine is also required for blood cell manufacture and protects tissues against damage caused by radiation and heavy metals.