What is Allergy? Allergy is a sensitivity to a specific substance called an allergen, which elicits a reaction and is usually contacted through skin, lungs, swallowing, or ingestion. An allergic response is a collection of symptoms caused by an immune reaction to substances that do not trigger such a response in most people. The immune system normally protects the body against harmful substances such as bacteria, viruses and toxins. Allergies occur when the immune system reacts to substances (allergens) that are generally harmless and in most people do not cause an immune response.