Monoclonal B-cell lymphocytosis | |
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Other names | monoclonal lymphocytosis of undetermined significance |
Specialty | Hematology, oncology |
Symptoms | None |
Complications | May progress to chronic lymphocytic leukemia or certain lymphoma types; increased risk of developing non-hematologic cancers, serious infections, and kidney disease |
Duration | chronic |
Types | CLL/SLL, atypical CLL/SLL, non-CLL/SLL, and MBL-MZ |
Monoclonal B-cell lymphocytosis (MBL) is an asymptomatic condition in which individuals have increased blood levels of particular subtypes of monoclonal lymphocytes (i.e. an aberrant and potentially malignant group of lymphocytes produced by a single ancestral cell). This increase must persist for at least 3 months.[1] The lymphocyte subtypes are B-cells that share certain features with the abnormal clones of lymphocytes that circulate in chronic lymphocytic leukemia/small lymphocyte lymphoma (CLL/SLL) or, less frequently, other types of B-cell malignancies. Some individuals with these circulating B-cells develop CLL/SLL or the lymphoma types indicated by their circulating monoclonal B-cells. Hence, MBL is a premalignant disorder[2]
In 2017, the World Health Organization (WHO) reclassified MBL as a distinct entity in which individuals have: 1) an excessive number of circulating monoclonal B-cells; 2) lack evidence of lymphadenopathy, organomegaly, or other tissue involvements caused by these cells; 3) no features of any other B cell lymphoproliferative disease such as one of the B-cell lymphomas; and 4) evidence that these cells have either a CLL/SLL, atypical CLL/SLL, or non-CLL/SLL phenotype based on these cells' expression of certain marker proteins.[3][4] A fourth MBL phenotype, monoclonal B-cell lymphocytosis-marginal zone (i.e. MBL-MZ) appears to be emerging as a distinct form of non-CLL/SLL MBL.[2]
MBL consist of two groups: low-count MBL has blood B-cell counts <0.5x9 cells/liter (i.e. 0.5x9/L) whereas high-count MBL has blood B-cell counts ≥0.5x9/L but <5x109/L.[5] While low-count MBL does not progress to a malignant disease, high-count MBL does so at a rate of 1-2% per year.[3] MLP-MZ is an exception to this rule in that it s usually associated with B-cell counts >3x109/L and all cases, regardless of B cell counts, have a somewhat higher risk of progressing to a malignant stage.[6]
The incidence of all MBL phenotypes increases with age and is strikingly high in the elderly. Below age 40, MBL's incidence is <1% of the general population in most countries but above this age it is found in ~10% of all individuals. The disorder's incidence in individuals >90 may be as high as 75%. Age along with B-cell blood counts, MBL phenotype, and certain genomic abnormalities in the monoclonal B cells are critical considerations in evaluating the clinical implications of MBL and its need for management.[2]