There are several systems of classification of Acute lymphoblastic leukemia. The classification is commonly used to determine treatment and predict the prognosis of the cancer.

French-American-British (FAB) classification

The French-American-British (FAB) classification that was used commonly earlier includes:-

• L1 – Around 25 to 30% of adult cases and 85% of childhood cases of ALL are of this subtype. In this type small cells are seen with:-
  • regular nuclear shape
  • homogeneous chromatin
  • small or absent nucleolus
  • scanty cytoplasm
• L2 – Around 70% of adult cases and 14% of childhood cases are of this type. The cells are large and or varied shapes with:-
  • irregular nuclear shape
  • heterogeneous chromatin
  • large nucleolus
• L3 – This is a rarer subtype with only 1 to 2% cases. In this type the cells are large and uniform with vacuoles (bubble like features) in the cytoplasm overlying the nucleus.

This classification was abandoned by the World Health Organization because the L1 and L2 subtypes could not be differentiated in terms of clinical symptoms, prognosis and genetic abnormalities. The mature B-cell ALL or L3 type is now classified as Burkitt's lymphoma/leukemia.

Revised version of FAB

WHO proposed a classification of ALL that was to be the revised version of the FAB classification. This used the immunophenotypic classification that includes:-

• Acute lymphoblastic leukemia/lymphoma or formerly L1 and L2 – this has subtypes including:-
  • Precursor B acute lymphoblastic leukemia/lymphoma – this has genetic subtypes including t(12;21)(p12,q22) TEL/AML-1, t(1;19)(q23;p13) PBX/E2A, t(9;22)(q34;q11) ABL/BCR and T(V,11)(V;q23) V/MLL
  • Precursor T acute lymphoblastic leukemia/lymphoma
• Burkitt's leukemia/lymphoma or formerly L3
• Biphenotypic acute leukemia

For immunophenotyping and classification of ALL a TdT assay and a panel of monoclonal antibodies for T cell and B cell associated antigens are used.

Immunophenotypic categories of acute lymphoblastic leukemia (ALL)

ALL can also present in variant forms or in different forms. This includes ALL with cytoplasmic granules, aplastic form of ALL, ALL with eosinophilia, secondary ALL and relapse of ALL or lymphoblastic leukemia.