Hay fever is caused by an allergic reaction of the immune system to harmless substances called allergens, most commonly pollen from trees, grasses, and weeds. When a person with hay fever breathes in pollen, their immune system mistakenly identifies it as a threat and releases chemicals like histamine, which lead to symptoms such as sneezing, itching, congestion, runny nose, and watery eyes. Other allergens that can trigger hay fever include dust mites, pet dander, mold, and certain foods. This overreaction of the immune system to these allergens causes inflammation of the mucous membranes in the nose and eyes.