Histidine is a semi-essential amino acid, which means your body normally makes as much as it needs. Like most other amino acids, histidine is used to make proteins and enzymes. The body also uses histidine to make histamine, the culprit behind the swelling and itching you feel in an allergic reaction. Although histidine is not required in the diet, histidine deficiencies can occur during periods of very rapid growth. Dairy products, meat, poultry, fish, and other protein-rich foods are good sources of histidine (see: Wikipedia).