Histamine is a ubiquitous messenger molecule released from mast cells, enterochromaffin-like cells, and neurons. Its various actions are mediated by a family of histamine receptors, which are a subset of the G protein-coupled receptor superfamily. Histamine H4 R is a histamine receptor that is predominantly expressed in hematopoietic cells. Histamine H4 R is thought to play a role in inflammation and allergy reponses.