The immune response that protects us from pathogens and malignancy is directed at the individual constituent molecules of the invading cells, a response specific for discrete sections of each of these molecules. A unique language is used to describe these individual targets, called epitopes or antigenic determinants. Understanding this language is necessary to better appreciate the character of the challenge and the nature, efficiency, and evolution of the response. As we determine the immune mechanisms of many diseases, e.g., autoimmunity due to molecular mimicry, it is crucial that physicians be able to interpret the immunological literature.Proper interpretation of serological testing, e.g., the use of Western blot to confirm/corroborate less specific tests like ELISA, depends in part on a full understanding of how the specificity of antibodies are determined. Novel therapeutic strategies, e.g., use of intravenous gammaglobulin to alter immune control networks, are being developed; only with an appreciation of the humoral immune response can the clinician make use of these new approaches that seek to manipulate the immune system. The terminology used in defining antigens, the molecules that are the target of the immune response, and the structure of immunoglobulins, and how these structures determine and influence function, are reviewed. This paper serves as a start, a "jumping off" point, for further description of the immune response in later papers in this series.To be successfully immersed in or introduced to immunology, it is important to "speak the language." So, consider this a refresher course, not to recall high school Spanish but medical school immunology.