The CD5+ B-cell.

In the last two decades, many efforts have been made to better understand the biology of B-lymphoproliferative disorders through the knowledge of physiology and function of the postulated normal counterpart. The follicular mantle B-cells express a typical CD23+ IgM+ IgD+ phenotype and surround the germinal center area in secondary lymphoid organs. CD5+ B-cells with FM phenotype can be isolated from different sources and all share similar morphologic, phenotypic and functional features (small cells, scanty nucleus/cytoplasm ratio, unmutated VH genes, response to polyclonal activators but not to T independent antigens, production of "natural" antibodies). While the CD5+ B-cells predominate in fetal life, their number decreases with age. However, the CD5+ B-cells have been demonstrated to increase again in elderly both in man and mouse. This finding may explain the incidence of B-CLL and of MCL that are believed to represent the malignant transformation of the normal CD5+ B-cells, among elderly and middle aged individuals, respectively.