Histamine H4 receptor (HRH4) (Nakamura et al. 2000) is highly expressed in bone marrow and white blood cells. It is also found in other tissues such as colon, liver, lungs and thymus. HRH4 mediates mast cell chemotaxis. Both the H3 and H4 receptors mediate their actions by coupling with the G protein alpha-i subtype, decreasing intracellular cAMP.