FAB classification

• ALL-L1: MPO-negative, with small cells predominating. Cells have a high nuclear–cytoplasmic (N/C) ratio (scant amount of cytoplasm), regular nuclear borders, and inconspicuous nucleoli. Tdt is usually positive

• ALL-L2: MPO-negative heterogeneous population, often with larger blasts. The cells have a low N/C ratio (moderate amount of cytoplasm), with irregular nuclear borders and prominent nucleoli. Tdt is usually positive

• ALL-L3, Burkitt type: MPO-negative, homogeneous population of large blasts. The cells have a moderate amount of deeply basophilic cytoplasm and prominent cytoplasmic vacuolation. The nuclei are regular, with one or more prominent nucleoli. The blasts are Tdt-negative and may be associated with t(2;8), t(8;14), or t(8;22) chromosomal abnormalities

Immune phenotype classification

ALL is also classified on the basis of the cell surface immune phenotype:

• T-ALL

• B-ALL, also designated as mature ALL

• Pre-B-ALL

• Pre-pre-B-ALL (or pro-B-ALL)

Note: “Null ALL” (CD10-, non-B, non-T, with expression of early B-cell antigens, e.g., CD19) predominantly represents pre-pre-B-ALL. The term “null ALL” is no longer in use.

Many conventional chemotherapy protocols for ALL currently stratify treatment according to risk status. This status, in turn, is determined by the immune phenotype and cytogenetic abnormality present.