Histidine Decarboxylase is an enzyme that catalyzes the conversion of histidine, an amino acid, into histamine, a biogenic amine. This reaction involves the removal of a carboxyl group from histidine, which is essential for the production of histamine, a compound involved in immune responses, gastric acid secretion, and neurotransmission. Histidine decarboxylase is primarily found in mast cells and neurons. Its activity is crucial for maintaining proper levels of histamine in the body. Dysregulation of this enzyme can lead to various health issues, including allergies and gastrointestinal disorders.