An allergic reaction is the body’s way of responding to an “invader.” When the body senses a foreign substance, called an antigen, the immune system is triggered. The immune system normally protects the body from harmful agents such as bacteria and toxins. Its overreaction to a harmless substance (an allergen) is called a hypersensitivity reaction, or an allergic reaction. Dr. Carlo Oller Board Certified Emergency Physician Please visit my website, www.DrER.tv make sure you subscribe, comment, and share! That is the best way to show your support.