HRH4 and Allergy: 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.