Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3 form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, it has now been shown in longer-term studies to be essential for adults also. It is encoded by the codons CAU and CAC.
Histidine was first isolated by Albrecht Kossel and Sven Gustaf Hedin in 1896. The name stems from its discovery in tissue, from ἱστός histós "tissue". It is also a precursor to histamine, a vital inflammatory agent in immune responses. The acyl radical is histidyl.