H₁ antagonist

noun

ˈāch-ˈwən- 

variants or H₁ receptor antagonist also H₁ blocker

: any of numerous drugs (such as fexofenadine or loratadine) that bind competitively with histamine to H₁ receptors on cell membranes and are used to relieve allergic symptoms and variously as sedatives, antiemetics, and anticholinergics

H₁ antagonists prevent and relieve the sneezing, nasal and ocular itching, rhinorrhea, tearing, and conjunctival erythema of the early allergic response to antigen, but they are less effective for the nasal blockage characteristic of the late allergic reaction.

—F. Estelle R. Simons and Keith J. Simons, The New England Journal of Medicine