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