Causes and Risks Allergies are caused by hypersensitivity of the immune system, leading to a misdirected immune response . The immune system normally protects the body against harmful substances such as bacteria, viruses, and toxins . Hay fever usually involves an allergic reaction to pollen. However, people may develop an allergic reaction to other particles in the environment.
Many people are allergic to mold.
Mold spores are carried in the air and may be present all year long. Outdoors, mold lives in the soil, on compost, and on damp vegetation. Dust is another common allergen . Mites, including small fragments of dead mites, are the primary causes of dust allergy. Many people are allergic to animals.