Leukemia is a cancer originating in the body’s blood-forming tissues. It is characterized by the uncontrolled production and accumulation of abnormal white blood cells that fail to mature or function correctly. These cancerous cells crowd out healthy cells—red blood cells, platelets, and normal white blood cells—leading to complications like anemia, bleeding, and increased susceptibility to infection.
The Dual Classification System
Physicians classify leukemia using a dual system based on two primary biological characteristics of the cancer cells. The first classification addresses the speed at which the disease progresses: acute or chronic. Acute leukemia involves a rapid increase of immature, non-functional blood cells, known as blasts, which multiply quickly and require immediate treatment. Chronic leukemia progresses much more slowly, often over months or years, and involves cells that are generally more mature.
The second classification factor identifies the specific type of white blood cell that turns cancerous, categorizing the disease as either myeloid or lymphoid. Lymphoid (or lymphocytic) leukemia affects the progenitor cells that develop into lymphocytes, which are involved in the immune system. Myeloid (or myelogenous) leukemia affects the myeloid cells, which are precursors that normally mature into red blood cells, platelets, and other white blood cells. Combining these two classification axes—speed of progression and cell origin—results in the four major categories of leukemia.
The Four Primary Types of Leukemia
Acute Lymphocytic Leukemia (ALL)
Acute Lymphocytic Leukemia (ALL) is defined by the rapid, uncontrolled growth of immature lymphoid cells, or lymphoblasts, in the bone marrow and blood. This form is the most common type of leukemia diagnosed in children, though it also affects adults, particularly those over the age of 65. The disease progresses quickly and requires intensive treatment.
Acute Myeloid Leukemia (AML)
Acute Myeloid Leukemia (AML) is characterized by the fast accumulation of abnormal myeloid blasts that fail to mature into functional red blood cells, platelets, or certain white blood cells. AML is the most common type of acute leukemia found in adults, with the majority of diagnoses occurring in older individuals, typically those over the age of 65. Due to its aggressive nature, AML requires immediate initiation of therapy.
Chronic Lymphocytic Leukemia (CLL)
Chronic Lymphocytic Leukemia (CLL) is a slow-growing cancer involving B-cell lymphocytes, which are a specific type of immune cell. This is the most common form of chronic leukemia in adults, frequently affecting people during or after middle age. Individuals with CLL may not show noticeable symptoms for years and can sometimes be monitored for extended periods without immediate treatment. The abnormal B-cells eventually build up, crowding out healthy cells and impairing immune function.
Chronic Myeloid Leukemia (CML)
Chronic Myeloid Leukemia (CML) is a slowly progressing cancer that originates in the myeloid cells, primarily affecting adults. A distinct genetic abnormality, known as the Philadelphia chromosome, is present in nearly all cases of CML, which fuses two genes to create an oncogenic protein. This form of leukemia is often discovered incidentally during routine blood tests, as patients may experience few or no symptoms early in the disease’s course. CML typically responds well to targeted therapy that specifically inhibits the protein produced by the Philadelphia chromosome.
Specialized and Rare Subtypes
While the “Big Four” represent the most common classifications, the overall number of distinct leukemias is much larger due to specialized and rare subtypes. These less common forms are identified by detailed analysis of the cell’s physical characteristics, genetic mutations, and specific protein markers. This level of sub-classification is important because certain subtypes require significantly different treatment approaches compared to their broader category.
Acute Promyelocytic Leukemia (APL)
Acute Promyelocytic Leukemia (APL) is a specific subtype of AML defined by a translocation between chromosomes 15 and 17. APL can cause life-threatening bleeding complications, but it responds exceptionally well to targeted, non-chemotherapy drugs like all-trans retinoic acid (ATRA) and arsenic trioxide. This specific genetic marker dictates a different initial treatment protocol than other AML subtypes.
Hairy Cell Leukemia (HCL)
Hairy Cell Leukemia (HCL) is a rare, slow-growing cancer of the B-lymphocytes, named for the fine, hair-like projections visible on the cell surface. This form is often considered a distinct entity and is generally highly treatable.
Juvenile Myelomonocytic Leukemia (JMML)
Juvenile Myelomonocytic Leukemia (JMML) is a rare cancer that affects young children, typically those under the age of four. It is classified as an overlap syndrome between myelodysplastic and myeloproliferative disorders.