Determination of ALL Lineage

As previously mentioned, once it is determined that the patient has 20% or greater blasts, the next step is to establish whether they are lymphoblasts or myeloblasts. Pages 11 and 12 deal with morphological differences as well as using cytochemical stains. Also, histochemical staining for terminal deoxynucleotidyl transferase (TdT) can be useful in determining that the blast is of lymphoid lineage. TdT is an enzyme found in immature (developing) lymphocytes which functions to help synthesize their specific antigen receptors.

If it is found to be Acute Lymphoblastic Leukemia, the next important step is to discover whether the lymphocytes involved are T cells or B cells. This can be accomplished by immunophenotyping for "markers" - that is , cell surface proteins that are specific for either B cells or T cells. Flow cytometry uses fluorescently stained antibodies specific for these markers. Using this technique, we can not only determine B or T cell lineages, but we can also count the numbers in a given quantity of blood or bone marrow.

These membrane proteins or markers usually have numbers prefaced by "CD" for Cluster of Differentiation. Among the many markers that can be detected, finding CD3 can identify the cell as a T cell and CD19, CD20, and CD22 will indicate B cell lineage. Table 11 shows some examples of CD markers for different cell lines.