These mature lymphocytes are increased markedly in number. They are indicative of chronic lymphocytic leukemia, a disease most often seen in older adults. This disease responds poorly to treatment, but it is indolent.

CLL is defined by more than 5000/microliter B lymphocytes in peripheral blood marking with CD23 and CD5. Over half of patients are diagnosed at an early, asymptomatic stage, without lymphadenopathy, splenomegaly, cytopenias, or autoimmune phenomena. With counts >10,000/microliter, progression and severity of CLL become more likely.



The term monoclonal B-cell lymphocytosis (MBL) is reserved for persons with <5000/microliter monoclonal B cells, and without features of a proliferative disorlder, such as lymphadenopathy, splenomegaly, cytopenias, or autoimmune phenomena. Such incidental mild monoclonal B cell populations in peripheral blood increase in prevalence with age.