Olopatadine is an inhibitor of the release of histamine from the mast cell and a relatively selective histamine H1_antagonist that inhibits the in vivo and in vitro type 1 immediate hypersensitivity reaction including inhibition of histamine induced effects on human conjunctival epithelial cells. Olopatadine is devoid of effects on alpha adrenergic, dopamine and muscarinic type 1 and 2 receptors. Following topical ocular administration in human, olopatadine was shown to have low systemic exposure.