Define and discuss antigen. Antigens are bound to specific receptors called antibodies, antigen receptors. The antigens are recognized as "foreign" and are targeted by antibodies. They are bound to the antibodies and illicit an antigen-antibody reaction. The reaction attempts to destroy the invading antigen. Many antigens are classed as allergens, which will illicit and allergic response. This includes severe inflammation, nasal secretion, swelling, wheezing, increased heart rate, urticaria, pruritus, and angio edema. A sever allergic response may occur, called anaphylactic shock, accompanied by vascular collapse, shock, and respiratory distress. Often leading to death.