Allergy Overview Make an Appointment Find a Doctor Ask a Question Millions of people around the world are affected by allergies. The tendency to develop allergies or allergic reactions is genetically inherited, meaning allergies tend to run in families. If you have a personal or family history of allergies and are exposed to certain things, called allergens, in your environment, you may become allergic to some of those substances. When you have a repeat exposure to one of these substances (through breathing or touching), you may have an allergic reaction, which can involve itching, swelling or nasal congestion, among other symptoms. Allergic reactions vary widely in severity from person to person and can also be affected by seasonal and environmental factors. Animal allergy Cockroach allergy Dust mite allergy Insect sting allergy Food allergy Latex allergy Medication/Drug allergy Mold allergy Pollen allergy Allergic Conditions Allergic conditions and allergic reactions occur when the immune system misidentifies harmless foreign substances — called allergens — and reacts to them as if they were harmful. Learn about common allergens such as food and insect stings. These allergens can cause different types of allergies and related conditions that affect different parts of the body. In this section, learn about the most common allergic conditions and what types of allergens typically cause them. Types Allergic conditions include: Allergic asthma Anaphylaxis Hives (urticaria) Rhinitis ( nasal inflammation) Eczema (atopic dermatitis) Programs & Services Allergy Treatment Programs (Adult) Clinical Trials For more than 100 years, National Jewish Health has been committed to finding new treatments and cures for diseases. Search our clinical trials.