Leukemias
The leukemias are malignant clonal disorders of blood-forming (hematopoietic) cells that originate in the bone marrow and typically circulate in the peripheral blood. They are grouped along two axes that organize the whole field: the cell lineage involved (myeloid versus lymphoid) and the clinical and biological tempo (acute versus chronic). This area orients the reader to that framework and links to the major disease topics within it.
Definition
Leukemia is a clonal neoplasm arising from hematopoietic progenitor or mature lymphoid cells, characterized by uncontrolled proliferation and accumulation of abnormal leukocytes in the bone marrow and, usually, the blood, classified by cell lineage and by acute or chronic course.
Scope
This area provides an orienting overview of the leukemias as a family of hematologic malignancies and the classification logic that separates them. It covers how lineage and acuity define the principal categories and points to the detailed topic entries on acute leukemia diagnosis, chronic myeloid leukemia, chronic lymphocytic leukemia, disease transformation, and the laboratory techniques used to characterize leukemic cells. It is educational reference material, not clinical guidance.
Sub-topics
Key concepts
- Clonal hematopoiesis and the leukemic stem cell
- Myeloid versus lymphoid lineage
- Acute versus chronic course
- Blast cells and the blast threshold
- Cytogenetic and molecular drivers
- Immunophenotype and morphology
- Measurable (minimal) residual disease
Mechanisms
Leukemias arise when a hematopoietic cell acquires somatic genetic lesions — chromosomal translocations, fusion genes, and recurrent mutations — that confer a survival or proliferative advantage and block normal maturation. In acute leukemias, maturation arrest leads to accumulation of immature blasts; in chronic leukemias, the malignant clone retains partial differentiation and expands more gradually. The lineage of the founding clone (myeloid or lymphoid) and the specific molecular driver shape the disease phenotype and underpin modern classification (Arber et al., 2016; Khoury et al., 2022; Arber et al., 2022).
Clinical relevance
The leukemias account for a substantial share of hematologic malignancy and span a wide range of behavior, from indolent chronic disease to rapidly progressive acute illness. Understanding how they are categorized is foundational for interpreting hematology literature and laboratory reports. This entry describes disease classes at a reference level and is not a basis for individual diagnosis or treatment decisions.
Epidemiology
Collectively the leukemias occur across all ages, with acute lymphoblastic leukemia predominating in children and the chronic leukemias and acute myeloid leukemia rising in incidence with age. Distribution varies by subtype, sex, and geography, and the relative frequencies differ markedly between pediatric and adult populations (Dohner et al., 2015).
Evidence & guidelines
Classification of the leukemias is governed by periodically revised consensus systems, including the World Health Organization classification (2016 revision and the 2022 5th edition) and the 2022 International Consensus Classification, which integrate morphology, immunophenotype, cytogenetics, and molecular genetics (Arber et al., 2016; Khoury et al., 2022; Arber et al., 2022).
History
Leukemia was described as a distinct entity in the mid-nineteenth century, and the field was progressively reshaped by the recognition of lineage and acuity, the discovery of recurrent cytogenetic abnormalities, and ultimately the integration of molecular genetics into formal classification systems (Arber et al., 2016).
Related topics
Seminal works
- arber-2016
- khoury-2022
- arber-2022-icc
Frequently asked questions
- How are the leukemias classified at the broadest level?
- Primarily by two axes: the lineage of the malignant cell (myeloid or lymphoid) and the clinical tempo (acute, with accumulation of immature blasts, or chronic, with a more gradual expansion of more differentiated cells).
- What distinguishes acute from chronic leukemia?
- Acute leukemia features arrested maturation and a rapid accumulation of immature blast cells, whereas chronic leukemia involves a malignant clone that retains more differentiation and expands more slowly.