In this volume the author by no means attempted to give a comprehensive view of the myriad of recent developments in immunology. The author has attempted, after introducing sufficient background in chapter 1, to highlight certain areas of lymphoid cell biology which are given less attention in current thinking but which seem to the author to offer exciting prospects for research. In other cases, the important topics of transplantation and tolerance are looked at from points of view which are somewhat less conventional in the hope that these aspects may stimulate others to look at the yet unsolved problems in these areas. In making the selection of topics and indeed in the selection of references within each chapter, the contributors and the author have attempted to select those which offered what seemed to be the best sources of information. They do not wish to minimize the important contributions of others but are guided by constraints of brevity and cogent expression.