There are several different types of allergies currently affecting 1 in 5 Americans. An allergy starts when your immune system reacts to a substance that may not be harmful but recognized as a foreign allergen. An allergen can include pollen, mold, animal dander, certain foods, chemicals and skin irritants.
The immune system will produce antibodies against that particular allergen. When the body detects the allergen again, the antibodies begin to release histamine causing allergy symptoms. The response may inflame your sinuses, skin, airway or digestive system. Once you develop an allergy, you are more likely to develop another one.