An allergy is an overreaction of the immune system by normally harmless subjects called allergens. Allergies come in a variety of forms. The most common allergens are pollen, dust mites, pet dander, insect venom, and food. Most people will suffer from at least one allergic reaction at some point in their life. Some people may experience hives or rashes from various foods or plants, sneezing from dust or pollen, and itchy eyes from dander. Of course, triggers and symptoms are different for everyone.