Classification of ALL was first done using the FAB (French, American, and British) system, which categorized ALL based on cell characteristics and the behavior shown when certain staining techniques were performed. In light of new techniques that have become available such as flow cytometry and cytogenetics, the FAB system has been replaced by the WHO system, which differentiates cases of ALL according to phenotype.
- Precursor B-cell leukemia: We find recurring genetic anomalies, and 7 subtypes are defined:
- t(9;22)-BCR/ ABL 1
- t(v;11q23)-MLL rearrangement
- t (12;21) TEL-AML1 (ETV6 – RUNX 1)
- Hyperdiploid ALL
- Hypodiploid ALL
- t(5;14) IL 3 –IgH
- t(1;19) TCF-3 –PBX1
- Precursor T-cell leukemia: We find a distinct biological subtype characterized by absence or weak expression of CD5. This subtype is associated with poor prognosis when currently used ALL treatment approaches are used.
- Mature B-cell leukemia (Burkitt's leukemia): to distinguish between leukemia and lymphoma, the bone marrow is examined for blast infiltration (if this is over 20%, the disease is leukemia, while if it is under 20%, it is lymphoma).