Leukemia is a cancer of the blood and bone marrow. Unlike solid tumors that form lumps in organs like the breast, lung, or colon, leukemia produces large numbers of abnormal blood cells, most often white blood cells, that circulate through the bloodstream. Around 67,790 new cases are expected in the U.S. in 2026, and the overall five-year survival rate sits at about 68.6%.
How Leukemia Starts in Bone Marrow
Your bone marrow is the soft tissue inside your bones that acts as a factory for blood cells. It continuously produces the cells that will become white blood cells (which fight infection), red blood cells (which carry oxygen), and platelets (which help blood clot). Leukemia begins when some of these developing cells pick up DNA mutations that cause them to multiply out of control.
The result is a flood of abnormal cells that crowd out the healthy ones. Because these defective cells can’t do their jobs properly, people with leukemia often become vulnerable to infections, feel exhausted from low red blood cell counts, and bruise or bleed easily due to reduced platelets. This crowding effect is what drives most of the symptoms, regardless of which specific type of leukemia a person has.
The Four Main Types
Leukemia is classified along two axes. The first is which cell line is affected: lymphocytic leukemia starts in lymphocytes (a type of white blood cell involved in immune defense), while myelogenous leukemia starts in myeloid cells (a different group of white blood cells, plus the precursors to red blood cells and platelets). The second axis is speed: acute leukemia involves very immature cells called blasts that multiply rapidly, while chronic leukemia involves slightly more mature cells that grow slowly over months or years.
Combining those two axes gives the four primary types:
- Acute lymphoblastic leukemia (ALL) is the most common childhood leukemia. The immature lymphocytes multiply quickly, and the disease can progress within weeks without treatment.
- Acute myeloid leukemia (AML) is more common in adults. Like ALL, it involves fast-growing immature cells, but these originate in the myeloid line rather than the lymphoid line.
- Chronic lymphocytic leukemia (CLL) is the most common leukemia in adults overall. It progresses slowly, and some people live years before needing treatment.
- Chronic myeloid leukemia (CML) also develops gradually. It is strongly linked to a specific genetic abnormality called the Philadelphia chromosome.
What Makes It Different From Solid Tumors
Most cancers form a mass you can see on a scan or feel during an exam. Leukemia doesn’t work that way. Because the cancerous cells live in blood and bone marrow, there is usually no lump to find. Diagnosis typically comes from blood tests that reveal abnormal cell counts, followed by a bone marrow biopsy to confirm the type. This also means leukemia doesn’t “spread” the way a solid tumor metastasizes. The cancer is already in the bloodstream from the start, which is why it can affect the entire body so quickly in acute forms.
The Role of Genetics
Some leukemias are tied to specific chromosomal changes. The best-known example is the Philadelphia chromosome found in most people with CML. It forms when pieces of chromosomes 9 and 22 swap places, creating a shortened chromosome 22 that sends faulty instructions to cells, driving them to divide uncontrollably. Identifying this abnormality matters because targeted therapies can block the protein it produces, transforming CML from a fatal diagnosis into a manageable chronic condition for many patients.
Other genetic changes play roles in different subtypes. Acute leukemias often involve mutations that prevent immature blood cells from maturing normally, so they stay in a blast stage and keep dividing. These genetic details help doctors choose the most effective treatment approach for each individual case.
Known Risk Factors
Most people diagnosed with leukemia have no obvious cause, but several factors raise the risk. Exposure to high levels of ionizing radiation, the type found in nuclear fallout and certain industrial settings, is one of the best-established links. Repeated contact with benzene, a chemical in petrochemicals and tobacco smoke, is another recognized risk. Some chemotherapy drugs used to treat other cancers can, ironically, increase the chance of developing leukemia years later.
For children, the picture is slightly different. A mother’s exposure to pesticides or certain chemicals during pregnancy may raise the risk of ALL in her child, and early childhood exposure to those same chemicals has also been linked to higher rates. Certain inherited genetic syndromes carry increased leukemia risk as well, though these account for a small fraction of cases.
Rarer Forms of Leukemia
Beyond the four main types, several uncommon variants exist. Hairy cell leukemia, named for the hair-like projections on the cancerous cells when viewed under a microscope, is a slow-growing form that affects a small number of adults each year. It behaves differently from the major types and requires distinct diagnostic testing to distinguish it from conditions that look similar under the microscope, such as certain lymphomas that also involve the spleen.
Some of these rarer forms carry specific genetic markers that influence how well they respond to treatment. Patients with certain mutations in hairy cell leukemia, for example, tend to respond poorly to standard chemotherapy and need alternative approaches. This is part of a broader trend in leukemia care: treatment increasingly depends not just on which of the four main categories a person falls into, but on the specific genetic fingerprint of their disease.