Allergic conjunctivitis is the most common allergy affecting the eyes. The conjunctivae—the thin membranes covering the eyelids and the exposed surface of the eyes—is an active tissue of the immune system that responds to allergies caused by airborne particles, or allergens.

Allergic conjunctivitis can appear in two forms: seasonal and perennial. The seasonal version is much more common, and is related to exposure to specific airborne allergens, such as grass, tree and weed pollens and molds. The perennial form persists throughout the year and is usually triggered by dust mites, animal dander and feathers.