In peripheral tissues such as the spleen, thymus and bone marrow its levels of expression are altered in response to inflammatory stimuli.16 Several lines of evidence indicate that the H4R plays a key role in both pulmonary and intestinal inflammatory diseases (asthma, colitis), pruritus, allergies and other immunological disorders, which has led to the therapeutic development of novel H4R ligands. This evidence concerns the gene HRH4 and allergic disease.