Monoclonal B-cell lymphocytosis
Monoclonal B-cell lymphocytosis (MBL) is a condition that resembles chronic lymphocytic leukemia (CLL), but does not meet the criteria for CLL, and does not require treatment. However, CLL requiring treatment develops at the rate of 1.1% per year.
The definition of CLL includes >5,000 CLL-phenotype B-cell lymphocytes per cubic millimeter. Patients with <5,000 (but not 0) CLL-phenotype B-cell lymphocytes per mm³, and no symptoms of CLL, are diagnosed with MBL.
The term monoclonal means that all the B cells are derived from a single cell.
Little is yet known about the causes of MBL, but as it is a forme fruste of CLL the etiologies of these two conditions would be closely related. Genetic changes that can be found in CLL have also been found in MBL, and relatives of people with CLL have a much higher chance of having MBL (13% of first-degree relatives in one study).
One concern about MBL is related to blood transfusions. MBL was found in 0.14% of blood donors in one study. It is unknown whether blood transfusion can transmit MBL.
Using flow cytometry, monoclonal cells with cell surface markers similar to those in CLL can be detected in some healthy adults, who do not meet the criteria for CLL (i.e., >5,000 CLL-type lymphocytes per mm³). If the diagnosis of CLL is based on the B cell count rather than the total lymphocyte count (which includes both B and T cells), many patients formerly diagnosed with Rai Stage 0 CLL would instead be classified as having MBL. Molecular techniques can detect monoclonal B cell levels as low as 3-5 B cells/microliter (comparable to the amount of stem cells in peripheral blood). The term "monoclonal B-cell lymphocytosis" was proposed by a consensus committee in 2005 to indicate a monoclonal B cell population in a person with less than 5,000 B lymphocytes per milliliter (or 5.0 x 109 B lymphocytes/L), no enlarged lymph nodes or enlarged liver and/or spleen or other indications of a lymphoproliferative disorder.
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