In which type of leukemia is there typically an increase in immature lymphocytes?

Acute Lymphoblastic Leukemia (ALL) is characterized by the proliferation of immature lymphocytes, known as lymphoblasts, which are not fully developed. This leukemia primarily affects children, although it can occur in adults as well. In ALL, the bone marrow and blood are flooded with these immature cells, leading to symptoms that arise from a lack of normal blood cells.

In comparison, while Chronic Lymphocytic Leukemia (CLL) involves lymphocytes, it typically consists of mature but abnormal lymphocytes rather than immature forms. Acute Myeloid Myelomonocytic Leukemia (AMML), a subtype of Acute Myeloid Leukemia, involves myeloid cells and can also show immature forms, but it is not primarily related to lymphocytes. Chronic Myelogenous Leukemia (CML) primarily involves a type of myeloid cell called granulocytes, and while it has phases with immature cells, the hallmark is the presence of mature and abnormal granulocytes.

In summary, the defining feature of ALL is indeed the significant increase in immature lymphocytes, clearly differentiating it from other types of leukemia where such a feature is not predominant.