The term allergy (derived from antigen, a substance capable of giving rise to the production of antibody) was coined by Baron Clemens von Pirquet in 1906 to indicate a state of changed reaction to a foreign substance (which von Pirquet called an allergen). Allergens, von Pirquet said, differ from antigens, in that they can lead not to the production of antibody but to a condition of hypersensitivity. In his view, the term immunity should be restricted to those processes in which the introduction of a foreign substance into an individual does not cause a reaction that can be detected with medical tests; in such cases, individuals would be in a state of complete insensitivity. On that basis, it is apparent that, to understand and control allergic diseases, we must understand the immune system. In this century we have seen a remarkable expansion of our knowledge of the immune system, knowledge that has been used to gain insight into the relationship of the immune system to allergic diseases. Further advances in the ability to diagnose and treat allergic diseases depends largely on the application of this knowledge to the treatment of allergic people. Author’s Note Because the field of immunology is so broad, this chapter could only highlight a few of its salient features. For a comprehensive treatment of the field of immunology, the interested reader is referred to : Samter, Immunological Diseases, 2nd ed. (Boston: Little, Brown, 1971); Middleton et al., Allergy Principles and Practice (Saint Louis: C. V. Mosby, 1978); and Fudenberg et al., Basic and Clinical Immunology (Los Altos, Cal.: Lange Medical Publications, 1980).