Leukemia is a cancer originating in the body’s blood-forming tissues, primarily within the bone marrow. This condition involves the abnormal production of white blood cells, where genetic changes cause these cells to grow and divide uncontrollably. Over time, these abnormal cells can accumulate, crowding out healthy blood cells in the bone marrow. This disruption impairs the normal functions of the blood, affecting the body’s ability to fight infection, carry oxygen, and clot properly.
Understanding the Main Categories
Leukemia is classified based on two primary characteristics: its progression speed (acute or chronic) and the type of white blood cell affected. Acute leukemias develop rapidly, characterized by the quick proliferation of immature blood cells that cannot function correctly. These immature cells, often called blasts, quickly accumulate in the bone marrow, significantly reducing the production of healthy blood cells.
Chronic leukemias, in contrast, progress more slowly, sometimes taking months or even years for symptoms to become noticeable. In chronic forms, the abnormal cells are more mature, though still dysfunctional, and they multiply at a slower rate than in acute leukemia. The second classification factor considers the specific type of white blood cell from which the cancer originates: myeloid or lymphoid cells. Myeloid cells develop into red blood cells, platelets, and certain white blood cells like granulocytes and monocytes. Lymphoid cells, on the other hand, mature into lymphocytes, which are a type of white blood cell that plays a significant role in the immune system. These two classification systems combine to define the four major types of leukemia.
The Four Major Types
The combination of progression speed and cell type results in four primary forms of leukemia.
Acute Myeloid Leukemia (AML) is a fast-growing cancer that begins in the bone marrow and affects myeloid cells. The bone marrow produces an excessive number of immature myeloid cells, unable to mature into functional red blood cells, white blood cells, or platelets. These abnormal, rapidly dividing cells interfere with the production of healthy blood components. AML is the most common acute leukemia in adults, though it can also affect children.
Acute Lymphoblastic Leukemia (ALL) is also a rapidly progressing cancer that starts in the bone marrow, but it specifically affects lymphoid cells. This type involves the overproduction of immature lymphocytes, known as lymphoblasts, which do not develop into functional infection-fighting cells. These abnormal lymphoblasts quickly accumulate in the bone marrow and can spread into the bloodstream and other parts of the body. ALL is the most common form of leukemia diagnosed in children, but it can occur in adults as well.
Chronic Myeloid Leukemia (CML) is a slower-growing leukemia that originates in myeloid cells within the bone marrow. In CML, there is an uncontrolled growth of more mature, but still abnormal, myeloid cells. This type is frequently associated with the Philadelphia chromosome, a genetic abnormality resulting from a rearrangement between chromosomes 9 and 22. This change leads to an abnormal protein contributing to uncontrolled leukemia cell growth. CML typically affects adults, with symptoms potentially not appearing for years.
Chronic Lymphocytic Leukemia (CLL) is a slow-growing cancer that affects lymphoid cells, particularly B lymphocytes. This form of leukemia is characterized by the accumulation of mature but dysfunctional B cells in the blood, bone marrow, lymph nodes, and spleen. The abnormal cells do not function effectively and can gradually crowd out healthy blood cells over time. CLL is the most common chronic leukemia in adults, especially in Western populations, and it often occurs in individuals over the age of 55.
Other Less Common Forms
Beyond the four major types, other, less common forms of leukemia exist, as well as related conditions. One such example is Hairy Cell Leukemia (HCL), a rare, slow-growing chronic leukemia. HCL is characterized by the presence of abnormal B lymphocytes that have unique, hair-like projections on their surface when viewed under a microscope. These cells can accumulate in the bone marrow, spleen, and liver.
Myelodysplastic Syndromes (MDS) are a group of disorders that are closely related to leukemia but are not classified as leukemia themselves. In MDS, the bone marrow produces blood cells that are poorly formed or dysfunctional and do not mature properly. While MDS is distinct from leukemia, a significant proportion of MDS cases can progress to Acute Myeloid Leukemia (AML) over time.