The runny nose and puffy eyes that can signal an allergy are signs our body is overreacting to an otherwise harmless foreign invader. Our immune system’s rst line of defence is lymphocyte cells that move between tissues and blood vessels seeking out potentially harmful foreign cells. The rst time the lymphocytes meet a foreigner (or antigen) that they identify as dangerous they make an antibody called IgE, tailored for that antigen. Then the IgE molecules attach to other cells found in the bloodstream, lung tissues, skin, tongue and linings of gastrointestinal tract and nose. The next time the antigen enters the body, the cells with IgE attachments ght the invader by releasing chemicals such as histamine. The chemicals set off a string of reactions that irritate the tissues and lead to allergy symptoms.