Allergic conjunctivitis is an inflammation of the clear layer of skin that covers the eye and is triggered by an allergic reaction. It occurs when the eye comes into contact with an allergen and reacts by releasing histamine. This causes the blood vessels of the conjunctiva (a membrane that covers the white of the eye and inside of eyelids) to enlarge.

This is quickly followed by eye-related symptoms such as reddening of the eye, an itching sensation and tearing. These symptoms usually are accompanied by nasal symptoms, such as congestion, sneezing and an itchy nose. While non-allergic conjunctivitis often affects only one eye, allergic conjunctivitis usually affects both eyes at once.

There are several categories of allergic conjunctivitis:

• Seasonal allergic conjunctivitis (SAC). The most common form of allergic conjunctivitis, it occurs seasonally as plants pollinate and produce airborne allergens such as grass, tree and weed pollens, and molds. It is frequently a consequence of allergic rhinitis (hay fever) and is sometimes referred to as “hay fever conjunctivitis.”
• Perennial allergic conjunctivitis (PAC). A less common form of allergic conjunctivitis in which symptoms are experienced all year long. The main allergens involved in PAC are animal dander, dust mites and feathers. Though symptoms can occur throughout the year and usually are milder than those of SAC, they may worsen seasonally.
• Vernal conjunctivitis. A chronic form of conjunctivitis that occurs most often in the spring and fall. It has the potential to permanently damage vision and is much more likely to occur in males than in females, particularly those younger than 10 who also have a history of allergies. Vernal conjunctivitis exists in both allergic and non-allergic forms. Children often outgrow the condition by the time they reach early adulthood.
• Atopic keratoconjunctivitis. A type of allergic conjunctivitis that is associated with atopic dermatitis of the eyelids and face. Symptoms include redness, itchiness, burning, tearing, stringy or ropy discharge and swelling. Age of onset is usually in the teens through early 20s, and patients often have a history of allergies, especially allergic rhinitis and/or asthma.
• Giant papillary conjunctivitis. A type of conjunctivitis associated with contact lenses. This type of conjunctivitis is believed to be an allergic reaction involving proteins that stick to the surfaces of contact lenses and other prosthetic devices or surgical sutures. As the name implies, bumps appear on the inner surface of the upper eyelids. This may also have non-allergy-related causes.

An untreated bout of seasonal or perennial allergic conjunctivitis rarely leads to long-term complications. However, the problem can spread to other parts of the eye and cause inflammation of the iris, or colored part of the eye. Vernal conjunctivitis and atopic keratoconjunctivitis have the potential to damage sight.

Allergic conjunctivitis is often confused with other forms of conjunctivitis, commonly known as “pink eye.” Pink eye refers to conjunctivitis caused by a viral infection. Allergic conjunctivitis occurs as the result of an allergic reaction. The eyes contain a large number of mast cells, which are a part of the immune system. An allergy is the body's inappropriate or exaggerated response to a substance it believes is harmful.

Conjunctivitis can also be caused by other factors, including:

• Bacterial infection (bacterial conjunctivitis)
• Chemical irritation
• Eye injuries
• Other forms of irritation that do not involve the immune system
• Parasites (rare)
• Fungal infection (rare)

Allergic conjunctivitis tends to occur more frequently in individuals who have other types of allergies, particularly allergic rhinitis. Unlike bacterial and viral conjunctivitis, allergic conjunctivitis is not contagious.