Just like for allergies in the nose, the same stuff in the air (pollen, etc.) can cause eye allergies, which affect the conjunctiva (see Diagram: Eye — Anatomy). It virtually always involves both eyes, which may itch or feel gritty, but do not hurt (true pain suggests more serious illnesses). There’s usually clear tearing; yellow gunk usually piles up in the morning from overnight, but is rarely present during the day. The vision is normal (except it may feel blurry now & then due to tears, but certainly not blurry all the time).
Allergies can look the same as a Viral Conjunctivitis, which is an infection. We think “allergies” if there’s also a lot of sneezing. We’re sure it’s allergies if it has gone on for a month or more, or if in the past, the patient had similar symptoms lasting over a month (viruses don’t last that long). But at the beginning, it can be hard to tell the difference.
Treatment works well for allergies (nothing helps a virus much). Oral non-sedating antihistamine medications like loratadine (Clariten®), cetirizine (Zyrtec®), or fexofenadine (Allegra®) are best. Cromolyn eye drops do the same by preventing symptoms. Antihistamine eye drops can help with itching (ketotifen is over-the-counter).
Don’t use over-the-counter drops for redness, which work by cutting off circulation (not healthy at all). Ask a pharmacist for help. And don’t buy artificial tears, which are no more useful than tap water. The best medication for Allergic Conjunctivitis are steroid eye drops, but they’re quite dangerous, especially long-term. Ophthalmologists, or Optometrists in some states, prescribe them, but only an ophthalmologist should manage ongoing use. See Allergic Conjunctivitis (contact allergy) for allergies to substances which touch the eye.