This work is a natural history of the major histocompatibility complex written for anyone who is working on the MHC but wants to step back and review the complex in its entirety; for anyone who wants to be introduced to the MHC; or for anyone who is just curious about this much talked-about chromosomal region. By natural history is meant an all encompassing treatment of the MHC, both human and mouse, in its entire range and its details - but presented in a way that anybody can understand without consulting a variety of other sources. Most of the details are summarized in the form of tables whereas the text concentrates on generalizations and discussions. Thus, the book will be of interest to immunologists and geneticists seeking an overview of the subject and to graduate students in these disciplines. It will also be of value to technicians working in HLA-typing laboratories.