Histamine is a ubiquitous messenger molecule released from mast cells, enterochromaffin-like cells, and neurons. Its various actions are mediated by histamine receptors H1, H2, H3 and H4. Histamine receptor H4 belongs to the family 1 of G protein-coupled receptors and has the highest homology to histamine receptor H3 among known G protein-coupled receptors.The Histamine H4 receptor has been reported in human blood peripheral leukocytes, bone marrow, colon, liver, lung, smalll intestine, spleen, testis, thymus, tonsil, and trachea.