Allergic conjunctivitis is an inflammatory reaction evoked by an allergen. Common symptoms are irritation, epiphora (production of tears), redness, and swelling. The most important allergens are pollen (hay fever), dust (mites), pet hairs, and liquids for the eye.
Allergic conjunctivitis is usually due to a type I (IgE-mediated) hypersensitivity reaction. The allergen binds IgE and the immune complex triggers mast cell degranulation, releasing inflammatory mediators. Allergic conjunctivitides caused by liquids are generally type IV (cell-mediated) hypersensitivity reactions.