**3.1 Classification**

ALL is a hematologic malignancy with uncontrolled proliferation of lymphoblasts of B- or T-cell origin. ALL cases are clinically classified as B-cell precursor (BCP), mature B-cell, or T-cell types. BCP-ALL arises in B lymphocytes in the early stages of development in the bone marrow and affects 75–80% of adult patients. Mature B-cell ALL arises in more mature developing lymphocytes. This type of ALL is less common and accounts for around 3–5% of all adult cases. In around 20–25% of cases, ALL arises in developing T cells. This type of ALL can be further classified as early, mid, or late, depending on the maturity of the affected cell. T-cell ALL is commonly presented with a high white blood cell count and involvement of the central nervous system at diagnosis [81] (**Table 1**).
