This volume deals with the structure and function of molecules that have, during the last decade, turned out to have a central role in immune responses. Trans plantation antigens were discovered and characterized by Gorer about 50 years ago, and the biological basis for the unequalled complexity of their variability between individuals within a species, in spite of extreme conservation between species, was the subject of intense research and discussion for many years. During the days of belief in "immune surveillance" against spontaneously developing tumors, it was suggested that histoincompatibility between members of one species would prevent cancer from being a contagious disease and thus a threat to the species. Immunologists involved in human transplantation had to learn and care about the complexity, especially after 1967, when it was found that HLA antigens were the products of the human MHC. Rejection of HLA-identical sib kidney grafts was so rare, even in those days, that cases of rejection were described in scientific papers."

With this book we* want to address young graduate students, clini cians involved in transplantation, and technicians in transplantation immunology laboratories. The volume should give a comprehensive but basic, up to date introduction to the structure, function, and clinical importance of the HLA system. We believe that there is a need for such a survey, and think that the present level of our knowledge is an optimal occasion for publication. A significant number of ques tions have now been resolved, and our knowledge has reached a level of sophistication that provides the basis for additional questions and answers. Although the emphasis of this book is on the role of HLA anti gens in clinical transplantation, their involvement in other clinical contexts is also discussed. The main focus is on the human MHC an tigenic system, but MHC systems in other species are described as they contribute to our understanding of the structural and functional characteristics of HLA antigens. Some important issues related to laboratory techniques are also covered. The contributors have a close affiliation to the field of transplan tation immunology. A majority have even been playing important roles in unraveling the HLA system and its functions. We believe this has contributed significantly to the quality and clinical and practical relevance of the book. As editors, we drew up the principal guidelines and took care that the chapters can be read as separate entities, although this invariably results in some overlapping.