Histidine is one of the basic (by pH) amino acids due to its imidazole side chain. It is the direct precursor of histamine, one of the proteins involved in the immune response. Histidine is also an important source of carbon atoms in the synthesis of purines, one of the two groups of nitrogen bases that make up DNA and RNA. Histidine is needed to help grow and repair body tissues and to maintain the myelin sheaths that protect nerve cells. It also helps manufacture red and white blood cells and helps to protect the body from heavy metal toxicity. Histamine stimulates the secretion of the digestive enzyme gastrin and acts as a catalytic site in certain enzymes.
| Micromole per liter | µmol/L = mcmol/L = umol/L = µM/L = mcM/L = uM/L = micromol/L |
| Milligram per liter | mg/L = millig/L = milligram/L = mg/liter |
| Milligram per deciliter | mg/dL = millig/dL = milligram/dL |
| Milligram per 100 milliliters | mg/100mL = millig/100mL = milligram/100mL |
| Milligram percent | mg% = millig% = milligram% |
| Microgram per milliliter | µg/mL = mcg/mL = ug/mL = microg/mL = microgram/mL |