Allergic conjunctivitis (AC) is inflammation of the conjunctiva (the membrane that covers the white part of the eye) due to an allergy. Although allergens differ between patients, hay fever is the most common cause. The symptoms are redness (mainly due to vasodilation of the peripheral small blood vessels), edema (swelling) of the conjunctiva, itching, and increased lacrimation (tear production). When this is combined with rhinitis, the condition is known as allergic rhinoconjunctivitis (ARC).
Symptoms are due to the release of histamine and other active substances by mast cells, which stimulate blood vessel dilation, irritate nerve endings, and increase tear secretion.
Treatment of allergic conjunctivitis is by avoiding the allergen (e.g. avoiding flowering grass during the "hay fever season") and treating it with antihistamines, either topically (in the form of eye drops) or systemically (in the form of tablets). Antihistamines, mast cell stabilization drugs, and nonsteroidal anti-inflammatory drugs (NSAIDs) are generally safe and usually effective.