Allergic reactions do not happen the first time you come into contact with an allergen, but at a later point of contact. This is because the body’s immune system has to develop sensitivity to the allergen (see box) before you can become allergic to it.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.Allergy symptoms depend on your particular allergy, and can involve the airways, sinuses and nasal passages, skin, and digestive system.