[Lymphatic system and differentiation of B and T lymphocytes]

Most malignant tumours are classified according to their cellular origins. With regard to the classification of the malignant lymphomas, however, the application of this concept was not possible for a long period. Attitudes towards this changed rapidly with the discovery and description of the various cellular components of lymphoid tissue, and with the development of methods, which reliably distinguish the cells of the lymphoid system from each other. It has become apparent that the cellular origin of most lymphomatous diseases determines their respective morphological and clinical characteristics and, therefore, represents a sound basis for the classification of most lymphomas. This principle of classifying malignant lymphomas was initially applied by the Kiel Classification. Since itsí publication, further scientific discoveries have enabled us to close the gaps in our knowledge regarding the development of lymphoid tissue and cells. These advances were incorporated in the REAL Classification, and, subsequently, in the new WHO Classification. It is obvious, therefore, that an understanding of the malignant lymphomas and their classification requires a basic understanding of the differentiation of the cells of the lymphoid system. Consequently, a description of the components, the development and of the various developmental stages of the lymphoid cells will be briefly reviewed here, serving as a basis for the following chapters, which will summarise the characteristics of the different lymphomas. As the majority of the malignant lymphomas arise from B- and T-cells, these cell types will be concentrated upon in this review.