Catalyzes the biosynthesis of histamine from histidine. H(+) + L-histidine = CO2 + histamine Phosphorylation of brain HDC by cAMP-dependent protein kinase leads to enzyme inactivation. Amine and polyamine biosynthesis; histamine biosynthesis; histamine from L-histidine: step 1/1. Homodimer. A number of isoforms may be produced. Brain, glandular regions of the stomach, mast cells and fetal liver. May be post-translationally processed. A putative trans-splicing which involves HDC and SHBG gene regions produces a fusion protein expressed in fetal liver. Belongs to the group II decarboxylase family.