Histidine (symbol His or H) is an essential amino acid. It is used in the biosynthesis of proteins. It is a positively charged amino acid.
Originally it was thought essential only for infants. It has now been shown to be essential for adults also.[1] It is encoded by the codons CAU and CAC.
Histidine was first isolated by Albrecht Kossel and Sven Gustaf Hedin in 1896.[2] It is also a precursor to histamine, a vital inflammatory agent in immune responses.