Monoclonal B cell lymphocytosis (MBL)

Monoclonal B-cell lymphocytosis (MBL) is the presence of peripheral blood monoclonal B cells in numbers below 5x10^9/L in blood without any features of a B-cell lymphoproliferative disorder (1).

• the term MBL is used when there is an expansion of monoclonal B cells of uncertain clinical significance (1)
  • in population based studies which used highly sensitive flow cytometry techniques, 12% of the population and > 20% of persons older than 65 years were identified to have monoclonal B cells (2)
  • monoclonal B cells (MBC) tend to be higher in people whom peripheral blood MBC was detected from a medical work up (clinical MBL) than in people whom MBC was detected during population screening (3)
• usually the B cells are of CLL-phenotype but MBL patients without a CLL phenotype has been noted (1)
• it is seen frequently in first-degree family members of patients with CLL and in 5% of people older than 60 years who were tested (2)

MBL has been recognised as a potential precursor of CLL and, less frequently to other leukemic lymphoid neoplasms (2).

• the rate of progression of MBL to clinically recognizable CLL is around 1% - 2% per year B-cell count at diagnosis appears to be an important predictor of the outcome in clinical MBL
• several studies have pointed out that progression to CLL, treatment-free survival, and overall survival as a continuous variable can be predicted by the B-cell count (3)

Reference:

• 1. Rawstron AC et al. Monoclonal B-cell lymphocytosis and chronic lymphocytic leukemia. N Engl J Med. 2008;359(6):575-83
• 2. Campo E et al.The 2008 WHO classification of lymphoid neoplasms and beyond: evolving concepts and practical applications. Blood. 2011;117(19):5019-32
• 3. Gibson SE et al. Reassessment of small lymphocytic lymphoma in the era of monoclonal B-cell lymphocytosis. Haematologica. 2011;96(8):1144-52