A 4-year-old female is being evaluated for the sudden onset of multiple petechiae and bruises. She is found to have a peripheral leukocyte count of 55,000, 86% of which are small, homogeneous cells that have nuclei with immature chromatin. Indistinct nucleoli are also present. Initial tests on these immature cells are as follows: TdT, positive; PAS, positive; acid phosphatase, positive; and myeloperoxidase, negative. Based on these findings, the immature cells most likely originated from
Correct Answer: Lymphoblasts
Description: Acute lymphoblastic leukemia (ALL) is primarily a disease of children and young adults that is characterized by the presence of numerous lymphoblasts within the bone marrow. These malignant cells may spill over into the blood and other organs. In contrast to myeloblasts, lymphoblasts do not contain myeloperoxidase, but they do stain positively with the PAS stain or acid phosphatase stain and for the enzyme TdT. The French-American-British (FAB) classification of ALL divides ALL into three types based on the morphology of the proliferating lymphoblasts. L1-ALL, seen in about 85% of the cases of ALL, consists of small homogeneous blasts. L2-ALL, seen in only 15% of cases of ALL, but more common in adults, consists of lymphoblasts that are larger and more heterogeneous (pleomorphic) than L1 blasts. These cells may also contain nuclear clefts. The final type of FAB ALL is the L3 type, which is seen in less than 1% of the cases of ALL. This form is essentially the leukemic form of Burkitt's lymphoma. Like the malignant cells of Burkitt's lymphoma, these L3-ALL cells are large blasts with cytoplasmic vacuoles that stain positively with the oil red O lipid stain. In contrast to the FAB classification of ALL, the immunologic classification of ALL is based on the developmental sequence of maturation of B lymphocytes and T lymphocytes. First it is necessary to determine whether the blasts have B cell or T cell markers. Most cases of ALL are of B cell origin; that is, the lymphoblasts express both CD19 and DR. A few cases of ALL are of T cell origin; the lymphoblasts lack CD19 and DR and instead express T cell antigens CD2, 5, and 7. Many cases of T-ALL involve a mediastinal mass and are clinically similar to cases of lymphoblastic lymphoma. To subclassify B-ALL, first determine if surface immunoglobulin (sIg) is present. Mature B-ALL cells (L3-ALL or Burkitt's lymphoma) have surface immunoglobulin, which is not found in the other types of B-ALL. These mature cells typically lack TdT, which is a marker for more immature cells. Next determine if there is cytoplasmic u present. Cytoplasmic u chains are specific for pre-B ALL cells, which have a characteristic translocation t(1;19). The B cell ALL cells that lack both surface Ig and cytoplasmic u are called early pre-B-ALL and are separated into the CALLA (CD10)-positive and CALLA-negative types Acute Lymbhoblastic leukemia Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition.
Category:
Pathology
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