An allergy is any adverse (bad) reaction the body has to a substance that is normally considered harmless where immunoglobulin E (IgE) is involved. A substance that causes an allergic reaction is called an allergen; they are said to be 'harmless' as they only affect those who are allergic. An allergic reaction arises when the body fails to recognize the allergen as harmless and responds by producing antibodies as part of an immune response. The antibodies are made up of a protein called immunoglobulin, which exists in various forms within the body. IgE is the protein associated with allergic responses; elevated levels of IgE can be found (and tested for) in those who have suffered an allergic reaction. The allergy arises when the body comes into contact with the allergen for a second time, as during the first encounter lymphocyte cells make antibodies so, when the allergen enters the body a second time, more antibodies attach themselves to mast cells located near the skin. The antibodies then bind to the allergen activating the mast cells, causing them to break and release histamine. This results in the symptoms of an allergic reaction.