Although almost every part of the body can be affected with different symptoms by an allergy, the underlying mechanism is the same – a mistake by the immune system. We all have blood cells called lymphocytes which are the body’s home guard, constantly on the look-out for invaders such as bacteria and viruses. When lymphocytes come across something they recognize as not part of the body’s own proteins, they produce antigens, which in turn produce antibodies (known as IgE) to neutralize these foreign substances. When someone has an allergy, for reasons that are not yet clear, normally harmless substances like dust mites, cat hair or shellfish trigger off those antibodies, thus causing physical symptoms. The swelling and redness are caused because the IgE antigens cause cells in body tissue, called mast cells, to produce histamine and other chemicals to neutralize the foreign substances, which can quickly produce all sorts of effects from a runny nose or wheezy chest to skin rash or upset stomach. The IgE reaction provides the medical profession with a convenient marker of an allergy response.