Histamine is a ubiquitous messenger that mediates a large number of physiological processes. Its actions are mediated by four receptors named H1, H2, H3 and H4 receptors. The H1 histamine receptor is expressed primarily in lung, vasculature and brain where it mediates smooth muscle contraction, food intake and sleep, amongst other actions. Perhaps most notably, antagonists to this receptor are frequently prescribed to treat respiratory allergies.