Allergic conjunctivitis (AC) is inflammation of the conjunctiva (the membrane covering the white part of the eye) due to allergy. Although allergens differ among patients, the most common cause is hay fever. Symptoms consist of redness (mainly due to vasodilation of the peripheral small blood vessels), edema (swelling) of the conjunctiva, itching, and increased lacrimation (production of tears). If this is combined with rhinitis, the condition is termed allergic rhinoconjunctivitis (ARC). The symptoms are due to release of histamine and other active substances by mast cells, which stimulate dilation of blood vessels, irritate nerve endings, and increase secretion of tears. Treatment of allergic conjunctivitis is by avoiding the allergen (e.g., avoiding grass in bloom during "hay fever season") and treatment with antihistamines, either topical (in the form of eye drops), or systemic (in the form of tablets). Antihistamines, medications that stabilize mast cells, and nonsteroidal anti-inflammatory drugs (NSAIDs) are generally safe and usually effective. Signs and symptoms The conjunctiva is a thin membrane that covers the eye. When an allergen irritates the conjunctiva, common symptoms that occur in the eye include: ocular itching, eyelid swelling, tearing, photophobia, watery discharge, and foreign body sensation (with pain). Itching is the most typical symptom of ocular allergy, and more than 75% of patients report this symptom when seeking treatment. Symptoms are usually worse for patients when the weather is warm and dry, whereas cooler weather with lower temperatures and rain tend to assuage symptoms. Signs in phlyctenular keratoconjunctivitis include small yellow nodules that develop over the cornea, which ulcerate after a few days. A study by Klein et al. showed that in addition to the physical discomfort allergic conjunctivitis causes, it also alters patients' routines, with patients limiting certain activities such as going outdoors, reading, sleeping, and driving. Therefore, treating patients with allergic conjunctivitis may improve their everyday quality of life. Causes The cause of allergic conjunctivitis is an allergic reaction of the body's immune system to an allergen. Allergic conjunctivitis is common in people who have other signs of allergic disease such as hay fever, asthma and eczema. Among the most common allergens that cause conjunctivitis are: Pollen from trees, grass and ragweed Animal skin and secretions such as saliva Perfumes Cosmetics Skin medicines Air pollution Smoke Dust mites Balsam of Peru (used in food and drink for flavoring, in perfumes and toiletries for fragrance, and in medicine and pharmaceutical items for