Can Allergies Affect Just One Eye at a Time?

Can allergies affect just one eye but not the other?

The simple answer is no. Typical allergies such as hay fever, etc., are systemic and affect both eyes. It is possible that a small amount of allergen can get in one eye and cause a temporary problem, but this is highly unlikely. A red, swollen, tearing eye can be caused by tear duct obstruction, foreign body, exposed suture, ill-fitting contact lens, or small chalazion (stye). Look for causes other than allergy. If you have not done so already, consult an ophthalmologist.