The term "leukemia" comes from the Greek meaning  white blood. After centrifugation of a leukemia patient's blood, a white layer between the plasma and erythrocytes, known as the leukocrit (buffy coat), can be observed. The leukemias and leukocytes were both  named after the leukocrit.
Leukemia is a comprehensive term for malignant diseases of the hematopoietic stem cells and precursor cells, which usually includes an increase in the leukocyte count, increased leukemic blasts and the replacement of normal hematopoiesis. In acute leukemias, there are leukemic blasts that are blocked in an early differentiation stage, i.e. they cannot develop into mature blood cells. In chronic leukemias, mainly mature cells are increased.


Leukemias are classified first based on the clinical process into the chronic and acute forms, and secondly based on the specific involved cell line into myelocytic and lymphocytic forms. Four basic types of leukemias result:
- Acute myelocytic leukemia (AML)
- Acute lymphocytic leukemia (ALL)
- Chronic myelocytic leukemia (CML)
- Chronic lymphocytic leukemia (CLL)


Clinical picture:
The symptoms of acute leukemias typically develop within a few days or weeks. Symptoms occur due to the failure of normal hematopoiesis which results in anemia with weakness and exercise intolerance, neutropenia with a tendency toward infection and fever and thrombocytopenia with bleeding.
In the case of the chronic leukemias, the patients are normally asymptomatic for an extended period of time and the diagnosis is often made by chance. This particularly applies to chronic lymphocytic leukemia.


In acute leukemias, immature precursor cells (leukemic blasts) predominate in the peripheral blood picture. In addition, few mature cells can be found. Intermediate stage cells are absent.
The chronic forms of leukemia have typical blood counts. In chronic myelocytic leukemia, all stages of  myelopoiesis are present in the peripheral blood as well as an increase of basophils, eosinophils and platelets. In chronic lymphocytic leukemia, there are uniform mature lymphocytes and smudge cells present.
Establishing a diagnosis for leukemia is based on morphology, immunophenotyping, cytogenetics and molecular diagnostic tests.

Historically, acute leukemias were divided by morphological criteria according to the FAB-classification (FAB = French, American, British) in different subgroups. This classification was replaced by the WHO-classification of hematological malignancies in which besides morphologic criteria mainly genetic and molecular criteria are used.