When a person comes in contact with an allergen, an allergic reaction can start. Allergens can be inhaled, eaten, injected, or contacted by the skin. Allergens cause body cells to release a substance called histamine that makes the body react in an allergic manner. For example, in hayfever, pollen allergens can cause violent, repetitive sneezing, release of watery fluids, itching, tissue swelling, blocked nasal passages, and a loss of smell and taste.
Pollen allergies can make eyes itchy, red, and watery.
Allergens that effect the lungs can cause the airways to shrink and expand many times quickly which makes breathing difficult.