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 t owards this changed rapidly with the discovery and description of the vario us cellular components of lymphoid tissue and with the development of metho ds, which reliably distinguish the cells of the lymphoid system from each o ther. It has become apparent that the cellular origin of most lymphomatous diseases determines their respective morphological and clinical characteris tics and, therefore, represents a sound basis for the classification of mos t lymphomas. This principle of classifying malignant lymphomas was initiall y applied by the Kiel Classification. Since its publication,further scienti fic discoveries have enabled us to close the gaps in our knowledge regardin g the development of lymphoid tissue and cells. These advances were incorpo rated in the REAL Classification, and,subsequently,in the new WHO Classific ation. It is obvious, therefore, that an understanding of the malignant lym phomas and their classification requires a basic understanding of the diffe rentiation 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 fo r the following chapters, which will summarise the characteristics of the d ifferent lymphomas. As the majority of the malignant lymphomas arise from B - and T-cells, these cell types will be concentrated upon in this review.