Hay fever. Illustration of excessive mucus production in the human nose after inhalation of airborne pollen (yellow). Hay fever (allergic rhinitis) is caused by an allergic reaction to proteins on the surface of pollen grains. The proteins trigger the body's immune system, leading to inflammation of the mucous membrane inside the nose and consequent sneezing, runny nose and congestion. The condition is seasonal, occurring in response to the release of pollen by certain plants. Tree pollen is most prevalent in spring, grass pollen in summer and weed pollen in late summer. Antihistamine and corticosteroid drugs may be given to reduce inflammation.