NOW Supplements L-Histidine 600 mg is an essential amino acid that is best known for its role in the production of histamine, an important mediator of immune system responses.* Histidine is also critical for normal brain function, as well as for the maintenance of a healthy skin barrier.* In addition, histidine plays important roles in muscle function, cardiovascular health, protection against free radicals, and the elimination of heavy metals from the body.*