*6.1.1 Morphology and immunophenotype*

In leukemic presentations, the marrow is hypercellular and packed with lymphoblasts, which replace the normal marrow elements, lymphoblasts have more condensed chromatin, less conspicuous nucleoli, and smaller amounts of cytoplasm that usually lacks granules. Histochemical stains show periodic acid-Schiff–positive cytoplasmic material. Immunostaining for terminal deoxynucleotidyl transferase (TdT), is positive in more than 95% of cases in addition to B-cell markers. B-ALLs are arrested at various stages of development. The lymphoblasts usually express the pan B-cell marker CD19/CD22/CD79a and the transcription factor PAX5. CD10 is expressed in common ALL and pre-B ALLs express in addition cytoplasmic IgM heavy chain (μ chain) and surface immunoglobulins in mature forms (**Figure 1**).
