Histamine, acts via distinct histamine H1, H2, H3, and H4 receptors, and regulates various physiological and pathological processes. Hence the activity of Histamine receptors is very crucial. They are members of G-protein coupled receptors superfamily. Thus, have seven transmembrane domains, extracellular N-terminal tail and a cytoplasmic C-terminus¹.

H1 receptor is excitatory and couples to Gq‐type proteins, activating a signaling cascade which eventually leads to an increased cellular concentration of Ca^{2+ 3}.

H1 receptor is implicated in type I hypersensitivity allergic reactions caused by numerous allergens¹. An allergic reaction occurs when allergens activate mast cells that release histamine, the released histamine binds to H1R and this causes an activation of a cascade that leads to vasodilation and vascular hyper-permeability, resulting in allergic inflammation.

H₁R is expressed in various tissues throughout body, including the airway, vascular smooth muscle, and brain². H₁R expressed in the brain plays an important role in the regulation of sleep-arousal cycle and memory by binding histamine where it acts as a neurotransmitter¹.