From the Manufacturer:
It has been suggested that Histidine is more of an Essential Amino Acid rather than Non Essential. It is required for optimum growth. Histidine is metabolised into the neurotransmitter Histamine which is used by the body in its response to foreign substances such as hayfever.
Histidine plays an important part in the production of Red and White Blood Cells. It also helps to maintain the Myelin Sheath which is the protective shield for nerves.
In the stomach histidine also helps in producing gastric juices, therefore people with a shortage of gastric juices or suffering from indigestion may also benefit from this nutrient.