MBL in blood transfusion. Monoclonal B-cell lymphocytes are common in healthy blood donors and their detection may affect donor eligibility. MBL is defined as <5000 monoclonal B cells per μL of blood and no lymph node involvement. Low-count MBL (low risk of development of CLL, although risk may be affected by other factors) has <500 cells per μL and clinical MBL (increased risk of CLL) has >500 cells per μL. SLL, small lymphocytic leukemia. Professional illustration by Marie Dauenheimer.