Antibodies attach to mast cells.
The mast cells degranulate and release histamine. The histamine attaches to receptors that trigger the allergic reaction of itching, tearing , redness, and swelling. Although avoidance of the irritant may be the best solution to the problem, this is not always possible. These drops constrict the superficial conjunctival vessels and somewhat reduce congestion and redness. The transient stinging of the drop may relieve some of the itching.