Allergies, which affect an estimated 50 million Americans, occur when the immune system reacts to things that are normally harmless, such as pollens, certain foods, or medicines. Seasonal allergies occur at specific times of the year, while perennial allergies occur year-round. It is not clear why one person develops allergies and another does not, although the tendency to be allergic may be inherited. Allergies are often treated with medication, reducing contact with allergens, or shots.