It goes to work on mast cells in the eye. (Mast cells house the histamines that make your eyes itch.) Pataday stabilizes the mast cells, preventing them from releasing histamines.
Pataday also blocks histamines that have already been released from attaching to the histamine (H1) receptors in the eye, breaking the chain of allergic reaction.
Less histamines means less allergy eye itch.