HRH4 and asthma: However, the discovery and pharmacological characterization of H4R ligands especially antagonists, (and, to a lesser extent H3R and even H2R ligands) on mast cells, eosinophils, and T cells demonstrates the possibility of its involvement in inflammatory conditions/symptoms such as atopic dermatitis (AD), asthma, allergic rhinitis, rheumatoid arthritis (RA), and pruritus in humans.