During the last five years the histological classification of the non-Hodgkin lymphomas has become a complex subject. Even experienced pathologists and clinicians find it increasingly difficult to make their way through the extensive and often confusing literature. The confusion is mainly due to the fact that different "schools" use different starting points for their classifica­ tion. Moreover, since many immunological problems related to classifica­ tion have not been completely resolved yet, even researchers who use the same approach do not always arrive at the same classification scheme. This book represents an attempt to find common elements in three most important classifications for non-Hodgkin lymphomas (the Rappaport classification, the Lukes-Collins classification and the Kiel classification) and to provide a scientific basis for the classification of this disease group. It therefore defines the two main groups of lymphocytes and their specific distribution pattern in the lymphoid organs and discusses the methods by which these cells and related tumors can be recognized. The various malignant B- and T-cell lymphomas are described in detail by expert authors, who deal not only with the morphological details, but also with the relevant clinical aspects. Finally, some related special topics are dealt with to complete the contents of this book. SECTION I BASIC CONCEPTS AND METHODOLOGY IN LYMPHOMA CLASSIFICA TION 1. HISTOPHYSIOLOGY OF NORMAL LYMPHOID TISSUE AND IMMUNE REACTIONS P. NIEUWENHUIS AND K. LENNERT INTRODUCTION Not so long ago (1) the lymphocyte was defined as:" . . .