Histidine

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. It is encoded by the codons CAU and CAC.
Histidine was first isolated by Albrecht Kossel and Sven Gustaf Hedin in 1896. It is also a precursor to histamine, a vital inflammatory agent in immune responses.