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.

Though CLL is a B cell proliferation, marking with CD19 and CD20, it is characterized by the presence of a T cell marker, CD5, as shown by flow cytometry here. This is a systemic disease, and organ involement outside of marrow, such as spleen and liver, is known as small lymphocytic lymphoma (SLL).

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.