An allergy is the immune system’s response to a foreign substance that’s not typically harmful to your body. These foreign substances are called allergens. They can include certain foods, pollen, or pet dander. They elicit a response from your immune system.

Your immune system’s job is to keep you healthy by fighting infection and other harmful pathogens. It does this by attacking anything it fears could put your body in danger. Depending on the allergen, this attack response may involve inflammation, sneezing, and a host of other symptoms.