The good acceptance of this textbook is an indication that it has served its purpose. The present edition has been prepared in order to cover the main progress achieved in the five years that have elapsed since the first edition. The structure of the book remains essentially the same but a con siderable amount of new material has been introduced, particularly in certain areas such as the genetics of immunoglobulins and T cell receptor, the regulation of the immune response, hypersensitivity reactions, and cellular immunology. Today, immunology is essential for biologists in general and in particular for physicians, veterinarians, and pathologists. The great progress and diversification that has taken place in the last few years is due to its enormous value both for the understanding of theoretical biology and for the practical resolution of biochemical, genetic, pathological, and biological problems. Greatly contributing to this progress have been relatively sophisticated techniques, such as immunofluorescence, radioimmune assay, transmission electron micro scopy, scanning electron microscopy, isoelectric focusing, quantitative cytofluorimetry, affinity chromatography, and techniques that allow separation of the different lymphocyte subpopulations. A potentially fabulous field was recently opened with the development of techniques for obtaining monoclonal antibodies by fusion of immunologically active lymphocytes with myeloma cells. These hybrid cells produce large amounts of monoclonal antibodies or other lymphocyte factors. The establishment of this hybridoma technology, that is already routine in most laboratories, is being used in the resolution of general biology problems, particularly in the study of the various cell surface molecules.