Leukaemia is a progressive, malignant disease of the blood-forming organs, characterised by distorted proliferation and development of leukocytes and their precursors in the blood and bone marrow. It can be classified as acute or chronic, according to the degree of cell differentiation (not the duration of disease), and as myelogenous or lymphocytic, according to the predominant type of cell involved (myeloid or lymphoid). Several sub-types have been identified. The exact cause of leukaemia is unknown, but genetic and environmental risk factors have been identified for many sub-types. Typical signs and symptoms might include fatigue, weight loss, fever, pallor, ecchymoses, petechiae, dyspnoea, dizziness, palpitations, bleeding, and recurrent infections, although each sub-type has its distinguishing features. Definitive diagnoses often require bone marrow biopsy and/or blood analysis.
A malignant clonal disease that develops when a B/T-precursor-stage lymphoid progenitor cell becomes genetically altered through somatic changes and undergoes uncontrolled proliferation. Acute lymphocytic leukaemia (ALL) is the most common leukaemia in paediatrics, accounting for up to 80% of leukaemias in this group and 20% of leukaemias in adults.  Clinical presentation is heterogenous and reflects biological subtype. Most cases present with signs and symptoms associated with cytopenias. Enlarged lymph nodes are also frequently the initial cause for seeking medical attention by the patient.  T-lineage ALL is characterised by an older age of onset, male sex preponderance, and inferior outcome in comparison with B-ALL.  The cytogenetic abnormality has prognostic implications.
A cancer of B lymphocytes. Failure of B lymphocytes to undergo maturation and full differentiation leads to a monoclonal population of dysfunctional but self-renewing B lymphocytes. These lymphocytes can infiltrate lymphatic tissues and haematopoietic organs such as the liver, spleen and bone marrow. A key risk factor is age over 60 years. Lymphadenopathy, splenomegaly (in 50% of cases), and shortness of breath and fatigue are key diagnostic factors.    It is diagnosed by FBC with differential, blood smear showing smudge cells, and flow cytometry.    Most cases are diagnosed on a routine FBC for an unrelated reason.
The clonal expansion of myeloid blasts in the bone marrow, peripheral blood, or extra-medullary tissues. Occurs predominantly in older adults.  Common findings are pallor, ecchymoses and petechiae. Acute promyelocytic leukaemia (APML) is a form of acute myelogenous leukaemia with distinct cytological and clinical features, which has specific management. 
A malignant clonal disorder of the haematopoietic stem cell that results in marked myeloid hyperplasia of the bone marrow.  May also be called chronic granulocytic leukaemia, chronic myelocytic leukaemia, or chronic myeloid leukaemia. Median age at presentation is around 53 years, and the only known risk is exposure to ionising radiation.   Many cases are asymptomatic but possible symptoms include fever, chills, malaise, weight loss, and night sweats. Around 75% of patients have splenomegaly;  all patients will have elevated WBC count. Diagnosis should be confirmed by the presence of the Philadelphia chromosome and/or the BCR-ABL rearrangement in peripheral blood or bone marrow cells. 
Blast crisis refers to the transformation of chronic myelogenous leukaemia (CML) from the chronic or accelerated phase to the blast phase. Anaemia, infections, abnormal bleeding, bone pain, and constitutional symptoms (night sweats, weight loss, fever) are common presenting complaints of blast-phase CML. The goal is to achieve a complete haematological remission or at least return to the chronic phase in order to perform stem cell transplant.
A B-cell malignancy characterised commonly by symptoms of fatigue, a markedly enlarged spleen and a distinctive histological appearance on peripheral blood smear and bone marrow biopsy. Hairy cell leukaemia (HCL) is characterised by B-cells with delicate cytoplasmic projections resembling hair. There is marked geographic variation in the frequency of HCL (also known as leukaemic reticuloendotheliosis) and it is relatively uncommon. The median age of patients is 50 years and HCL affects more men than women.  It often presents with abdominal discomfort or fullness. This is attributed to splenomegaly, which is present in around 60% to 90% of patients.   The disease is not curable. However, it is highly responsive to therapy and may be managed successfully for a decade or more.
Leukaemias may cause pancytopenia through decreased production, or increased destruction or sequestration, of blood cells.
Neutropenia contributes to some of the typical symptoms of leukaemia, including recurrent infections.
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This overview has been compiled using the information in existing sub-topics.
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