Allergic conjunctivitis can be caused by an allergy, such as an allergy to pollen (hay fever), house dust mites or cosmetics.
There are four types of allergic conjunctivitis:
seasonal allergic conjunctivitis - this affects both of your eyes and people often get it at the same time as hay fever
perennial allergic conjunctivitis - people with this type of allergic conjunctivitis have symptoms every day throughout the year in both eyes, often on waking each morning
contact dermatoconjunctivitis - this type of conjunctivitis can irritate your eyelids and it occurs most often in people who use eye drops
giant papillary conjunctivitis - this is common in people who use soft contact lenses, although it can also occur in people using hard contact lenses and after eye surgery
You might develop allergic conjunctivitis if you're allergic to plant pollens that are released into the air at around the same time each year. This is called seasonal allergic conjunctivitis or hay fever conjunctivitis.
Perennial (all year round) allergic conjunctivitis can be caused by house dust mites or animal fur.
Eye drops, cosmetics, and other chemicals can also cause allergic conjunctivitis - eye drops are the most common cause.
You can get a form of allergic conjunctivitis called giant papillary conjunctivitis if you use contact lenses, or after eye surgery.
If you have allergic conjunctivitis, try to keep away from whatever is causing the allergy.
For example if you're allergic to a cosmetic, don't use it again and try an alternative product (wait until your symptoms have gone before you try the new product).
It may be more difficult if you're allergic to pollen, but keeping windows and doors closed on days when the pollen count is very high may help to reduce your symptoms.
A cool compress (a facecloth soaked in cold water) may help to soothe your eyes.