Allergic conjunctivitis is a condition in which the eyes become inflamed due to an allergic reaction. It occurs when the immune system overreacts to a substance, such as pollen or pet dander, that comes into contact with the eye. This causes the blood vessels in the conjunctiva (the thin layer of tissue covering the white part of the eye) to swell and become irritated, leading to symptoms like itching, redness, tearing, and puffiness. Allergic conjunctivitis is a common condition that can be managed with over-the-counter medications or prescription treatments, depending on the severity of the allergy.