There are two main types of allergies: seasonal and perennial. Seasonal allergies occur at specific times of the year, while perennial allergies occur year-round. Allergies are the immune system's specific reaction to a normally harmless substance, such as pollens, certain foods, or medicines. It is not clear why one person develops allergies and another does not, although the tendency may be inherited. Allergies are often treated with medication, reducing contact with allergens, or shots.