The Classification of Lymphoid Leukaemias.

Advances in immunological and molecular techniques and a greater attention to peripheral blood morphology, bone marrow histology and lymph node pathology have resulted in a more objective classification of the lymphoproliferative disorders. This, in turn, has facilitated the understanding of the biology and the natural history of these diseases and helped to predict the response to therapy. Among the chronic B-cell disorders, two main groups of diseases can be distinguished: primary leukaemias and non-Hodgkin's lymphoma (NHL) in leukaemic phase. Distinct differences in morphological and histopathological features together with characteristic immunophenotypic profiles constitute the best basis for their precise diagnosis and classification. Because of their great heterogeneity some problems still arise, particularly in cases of low grade NHL and chronic lymphocytic leukaemia when the disease undergoes malignant transformation. Leukaemias of mature T cells are less common in Western countries. They can be distinguished from B-lymphoid disorders by standard immunological markers. Cytogenetic and DNA analyses are essential to establish T-cell clonality not shown by marker studies. The morphology of the neoplastic cells and the variable histopathological features together with epidemiological factors, such as the human retrovirus HTLV-1, are key elements to characterise and classify more precisely the various clinico-pathological entities included in this group.