Immunophenotyping involves the identification of membrane proteins that are specific for certain lineages and stages of development. All white blood cells undergo differentiation and maturation in the bone marrow (and thymus if they are T lymphocytes). Each stage and lineage will synthesize different surface proteins necessary for further development and function. These surface proteins are called CD (cluster differentiation) antigens or markers and are usually identified using flow cytometry. In flow cytometry, fluorescently labeled antibodies specific for a CD antigen are incubated with the white cells and then passed through the flow cytometer, counting the number of cells that possess that specific CD marker. The image below is a simplified diagram of the basic parts of a flow cytometer and how it works.
The patient's CD profile was:
CD19, CD22, CD79a, CD10 POSITIVE
CD7, CD5, CD3, Cytoplasmic, and surface IgM NEGATIVE
CD19, CD22, and CD79a are all expressed in B lymphoblasts. While CD3 and CD7 are expressed by developing T Cells. Thus, our patient is diagnosed with B Cell Acute Lymphoblastic Leukemia (ALL). Further, the patient's leukemic cells are not yet expressing IgM, the first immunoglobulin synthesized on the cell surface of developing B Cells, further identifying the cells as very early blasts. Thus, the patient is diagnosed with Precursor ALL, one of the most common forms of childhood leukemia.
