Leukemia is a cancer of the blood-forming cells, most often originating in the bone marrow, the spongy tissue inside bones responsible for producing blood cells. Leukemia causes the bone marrow to produce abnormal white blood cells that do not function properly and, because they multiply and survive longer than normal cells, gradually crowd out the healthy white blood cells, red blood cells, and platelets the body needs.
Despite the name suggesting an increase in white blood cell numbers, leukemia can sometimes present with a normal or even low white blood cell count, since it is the abnormal function and behaviour of these cells, not simply their number, that defines the disease.
Leukemia is not one disease but several, and the distinction between them matters enormously. Some forms progress rapidly and require treatment to begin within days of diagnosis; others progress so slowly that some patients are simply monitored for years before treatment becomes necessary at all. Treatment for certain types of leukemia has also advanced dramatically over the past two decades, transforming what was once a rapidly fatal diagnosis into a manageable, long-term condition for many patients.
All blood cells, including white blood cells, red blood cells, and platelets, originate from blood-forming stem cells within the bone marrow, which mature through a carefully regulated process into their final, functioning forms. Leukemia begins when a genetic mutation occurs within one of these developing blood cells, disrupting this normal regulation and allowing the abnormal cell to multiply uncontrollably.
As these abnormal cells accumulate in the bone marrow and bloodstream, they progressively crowd out the space and resources needed for normal blood cell production. This is what drives the core problems seen in leukemia: too few healthy red blood cells causes anaemia and fatigue; too few functioning white blood cells impairs the body's ability to fight infection, even when total white cell counts appear elevated on a blood test; and too few platelets impairs normal blood clotting, causing easy bruising and bleeding.
Leukemia is classified along two dimensions: how quickly it develops and progresses (acute or chronic), and which type of white blood cell is affected (lymphocytic or myelogenous). This produces four main types.
Acute Lymphoblastic Leukemia (ALL)
The most common childhood leukemia, though it also occurs in adults. Involves immature lymphocytes that multiply rapidly, requiring treatment to begin promptly after diagnosis.
Acute myelogenous leukemia (AML)
The most common acute leukemia in adults, involving immature myeloid cells. Progresses quickly and, like ALL, requires urgent treatment initiation.
Chronic lymphocytic leukemia (CLL)
The most common chronic leukemia in adults, involving more mature lymphocytes that accumulate gradually. Many patients feel entirely well for years and are simply monitored (“watch and wait”) before treatment is needed.
Chronic myelogenous leukemia (CML)
Involves more mature myeloid cells, typically progressing slowly over months to years, though it can eventually accelerate into a more aggressive phase if untreated. Almost always associated with a specific genetic change, discussed further below, which has made it one of the most successfully treated cancers in modern medicine.
The distinction between acute and chronic leukemia is not just about speed; it fundamentally changes how urgently treatment is needed.
In acute leukemia, immature blood cells multiply so rapidly that, if left untreated, the disease can become life-threatening within weeks. This is why acute leukemia is generally treated as a medical emergency from the point of diagnosis, with treatment typically beginning within days, not weeks, and often requiring hospital admission for the initial phase of chemotherapy and close monitoring.
Chronic leukemia behaves very differently. Because the abnormal cells are more mature and accumulate more slowly, some patients with early-stage chronic lymphocytic leukemia, for example, may not require any treatment for years, with regular monitoring alone being entirely appropriate. Understanding which category a diagnosis falls into is one of the first and most important pieces of information a patient receives, since it shapes the entire pace and urgency of what follows.
