Class A GPCRs which are activated by the biogenic amine histamine, and expressed throughout the body, to be specific, in smooth muscles, on vascular endothelial cells, in the heart, and in the central nervous system. H1 modulatesileum contraction, modulate circadian cycle, itching, systemic vasodilatation, and bronchoconstriction (asthma). H2 speed up sinus rhythm, stimulates gastric acid secretion, smooth muscle relaxation, inhibits antibody synthesis, T-cell proliferation and cytokine production. H3 functions as a neurotransmitter in CNS, Presynaptic autoreceptors. H4 mediate mast cell chemotaxis.