Allergic conjunctivitis is the direct exposure of ocular mucosal surfaces to the environment that causes an immediate hypersensitivity reaction in which triggering antigens couple to reaginic antibodies (IgE) on the cell surface of mast cells and basophils, leading to the release of histamines that causes capillary dilation and increased permeability and thus conjunctival injection and swelling.
Nerve endings are also stimulated causing pain and itching.
Seasonal allergic conjunctivitis is the most common form in temperate climates. It usually occurs and recurs at a certain period of the year and subjectively more severe than perennial allergic conjunctivitis.
Perennial allergic conjunctivitis manifests and recurs throughout the year with no seasonal predilection. It is most common in tropical climates.