The human immune system responds to antigens in the environment, says James E. Grove, O.D., F.A.A.O. Sometimes this re-sponse leads to inflammation within tissues. When it does, it�s called a �hypersensitivity reaction.� There are four classifications of hypersensitivity reactions, but the two most important to the eye are Type I and Type IV.

Type I reactions, such as allergic conjunctivitis, are caused an allergen that crosslinks the proper IgE molecules on the surface of a mast cell.

An example of a Type IV reaction is contact dermatitis resulting from ocular medications that are instilled into the eye. These delayed reactions occur when sensitized T lymphocytes interact with the antigen itself. The interaction makes the T lymphocytes release chemicals such as lymphokines. Usually within 24 hours of this reaction, the patient experiences erythema and other signs of perivascular inflammation.