Leukemia is a malignant condition involving the excess production of immature or abnormal leukocytes, which eventually suppresses the production of normal blood cells and results in symptoms related to cytopenias.
Malignant transformation usually occurs at the pluripotent stem cell level, although it sometimes involves a committed stem cell with more limited capacity for self-renewal. Abnormal proliferation, clonal expansion, aberrant differentiation, and diminished apoptosis (programmed cell death) lead to replacement of normal blood elements with malignant cells.
The American Cancer Society estimates that in the United States in 2020 there will be about 60,530 new cases of leukemia (of all types) in adults and children, and about 23,100 deaths.
Classification of Leukemia
The current approach to classifying leukemia is based on the 2016 World Health Organization (WHO) system (classification for hematopoietic neoplasms). The WHO classification is based on a combination of clinical, morphologic, immunophenotypic, and genetic features. Other less commonly used classification systems include the French-American-British (FAB) system, which is based on the morphology of the abnormal leukocytes.
Leukemias are commonly also categorized as
Acute or chronic: Based on the percentage of blasts or leukemia cells in bone marrow or blood
Myeloid or lymphoid: Based on the predominant lineage of the malignant cells
The four most common leukemias and their distinguishing features are summarized in the table Findings at Diagnosis in the Most Common Leukemias Findings at Diagnosis in the Most Common Leukemias 
.
For 2020, the American Cancer Society estimated the distribution of new US cases by leukemia type as follows:
Other leukemias: 8%
Acute leukemias
Acute leukemias Acute leukemias Leukemia is a malignant condition involving the excess production of immature or abnormal leukocytes, which eventually suppresses the production of normal blood cells and results in symptoms... read more consist of predominantly immature, poorly differentiated cells (usually blast forms). Acute leukemias are divided into acute lymphoblastic leukemia Acute Lymphoblastic Leukemia (ALL) Acute lymphoblastic leukemia (ALL) is the most common pediatric cancer; it also strikes adults of all ages. Malignant transformation and uncontrolled proliferation of an abnormally differentiated... read more 
(ALL) and acute myeloid leukemia Acute Myeloid Leukemia (AML) In acute myeloid leukemia (AML), malignant transformation and uncontrolled proliferation of an abnormally differentiated, long-lived myeloid progenitor cell results in high circulating numbers... read more 
(AML).
Chronic leukemias
Chronic leukemias have more mature cells than do acute leukemias. They usually manifest as leukocytosis with or without cytopenias in an otherwise asymptomatic person. Findings and management differ significantly between chronic lymphocytic leukemia Chronic Lymphocytic Leukemia (CLL) Chronic lymphocytic leukemia (CLL) is characterized by progressive accumulation of phenotypically mature malignant B lymphocytes. Primary sites of disease include peripheral blood, bone marrow... read more (CLL) and chronic myeloid leukemia Chronic Myeloid Leukemia (CML) Chronic myeloid leukemia (CML) occurs when a pluripotent stem cell undergoes malignant transformation and clonal myeloproliferation, leading to a striking overproduction of mature and immature... read more (CML).
Myelodysplastic syndromes
Myelodysplastic syndromes Myelodysplastic Syndrome (MDS) The myelodysplastic syndrome (MDS) is group of disorders typified by peripheral cytopenia, dysplastic hematopoietic progenitors, a hypercellular or hypocellular bone marrow, and a high risk... read more are a group of clonal hematopoietic stem cell disorders unified by the presence of distinct mutations of hematopoietic stem cells. They involve progressive bone marrow failure but with an insufficient proportion of blast cells (< 20%) for making a definite diagnosis of acute myeloid leukemia; 40 to 60% of cases evolve into acute myeloid leukemia.
Leukemoid reaction
A leukemoid reaction is a neutrophil count > 50,000/mcL (> 50 × 109/L) not caused by malignant transformation of a hematopoietic stem cell. It can result from a variety of causes, particularly other cancers or systemic infection. Usually the cause is apparent, but apparent benign neutrophilia can be mimicked by chronic neutrophilic leukemia or chronic myeloid leukemia.
Risk Factors for Leukemia
Risk of developing leukemia is increased in patients with
History of exposure to ionizing radiation (eg, post–atom bomb in Nagasaki and Hiroshima) or to chemicals (eg, benzene, some pesticides, polyaromatic hydrocarbons in tobacco smoke); exposure can lead to acute leukemias
Prior treatment with certain antineoplastic drugs, including alkylating agents, topoisomerase II inhibitors, hydroxyurea, and maintenance lenalidomide after autologous stem cell transplantation with melphalan-containing conditioning regimens for multiple myeloma; can lead to a type of acute myeloid leukemia called t-AML or therapy-related AML
Infection with a virus (eg, human T lymphotropic virus 1 and 2, Epstein Barr virus) can rarely cause certain forms of ALL; this is seen mainly in regions where such infections are common, such as Asia and Africa
History of antecedent hematologic disorders, including myelodysplastic syndromes Myelodysplastic Syndrome (MDS) The myelodysplastic syndrome (MDS) is group of disorders typified by peripheral cytopenia, dysplastic hematopoietic progenitors, a hypercellular or hypocellular bone marrow, and a high risk... read more and myeloproliferative neoplasms Overview of Myeloproliferative Neoplasms Myeloproliferative neoplasms are clonal proliferations of bone marrow stem cells, which can manifest as an increased number of platelets, red blood cells (RBCs), or white blood cells (WBCs)... read more , which can lead to AML
Preexisting genetic conditions (eg, Fanconi anemia Decreased red blood cell (RBC) production Anemia is a reduction in red cell mass or hemoglobin and is usually defined as hemoglobin or hematocrit > 2 standard deviations below the mean for age. Some authorities also consider a relative... read more , Bloom syndrome, ataxia-telangiectasia Ataxia-Telangiectasia Ataxia-telangiectasia results from a DNA repair defect that frequently results in humoral and cellular deficiency; it causes progressive cerebellar ataxia, oculocutaneous telangiectasias, and... read more , Down syndrome Down Syndrome (Trisomy 21) Down syndrome is an anomaly of chromosome 21 that can cause intellectual disability, microcephaly, short stature, and characteristic facies. Diagnosis is suggested by physical anomalies and... read more
, xeroderma pigmentosum, Li-Fraumeni syndrome) predispose to acute myeloid leukemia and acute lymphoblastic leukemia.
More Information
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