A body reacts with exaggerated response of the natural immune system. Hay fever is usually caused by an allergy to pollen. Many asthmatics are allergic to certain dusts or to microorganisms in fur or feathers. There are many kinds of allergies, producing a wide range of effects. Some people experience skin reactions, such as dermatitis, from drugs, cosmetics or household cleaners.
Some people suffer stomach upsets due to allergens in food such as nuts or shellfish.
Some people suffer mental imbalance due to allergens in common foods. A severe allergic reaction may produce shock, which is sometimes deadly. Many people experience various forms of allergic reactions. Almost anything may be an allergen. The skin may redden and blister, and the breathing may become strained.