Specific aspects of recognising lymphoid blast cells


Concepts to be aware of in ALL diagnosis

There are no reproducible criteria that allow B-ALL to be distinguished from T-ALL with confidence, and morphology is not used to classify ALL within the WHO classification. Nonetheless, it is useful to be able to recognise the characteristics of lymphoblasts in blood. In particular, morphological features may strongly suggest the presence of the important subtype of ALL: Burkitt lymphoma.

The FAB criteria remain helpful in recognition of blasts

The descriptions of lymphoblasts from the previous FAB classification (while no longer used in routine classification) define three separate morphological entities:

  • Small lymphoblasts with relatively regular nuclei and scanty blue cytoplasm. Chromatin may be partially condensed and nucleoli may be barely visible - resembling a normal lymphocyte in many ways. This type is particularly frequent in childhood (the FAB L1 subtype)
  • Medium lymphoblasts of variable size with irregular nuclei and lacy chromatin, one or more nucleoli are usually visible but may not be distinct. Cytoplasm is easily seen, and is usually weakly basophilic and agranular. Occasionally with infrequent azurophilic granules may be present (the FAB L2 subtype)
  • Medium to large lymphoblasts with fine or stippled nuclear chromatin, and often with prominent nucleoli. Characteristically, the cytoplasm is strongly basophilic and often vacuolated (the FAB L3 subtype is often associated with Burkitt lymphoma)

Image A: a small "L1" type blast cell (upper cell), together with a larger more typical lymphoblast (lower cell); Image B: a blast cell with "L2" morphology. Image C: an "L3" lymphoblast with marked basophilia and cytoplasmic vacuolation.