Leukemia is cancer that starts in blood-forming cells

Leukemia is a type of cancer that begins in the bone marrow—the spongy tissue inside your bones where blood cells are made. Instead of producing healthy blood cells in the right amounts, the bone marrow starts making abnormal white blood cells that multiply out of control. These leukemia cells crowd out normal blood cells, which means your body cannot carry oxygen properly, fight infections, or stop bleeding the way it should.

The disease develops gradually in some people and rapidly in others, depending on the type. Some forms are more common in children; others appear mainly in adults. What all types share is that the problem starts at the cellular level—a single cell's DNA gets damaged in a way that makes it divide endlessly instead of following the normal rules.

Key Takeaways

  • Leukemia starts when bone marrow produces too many abnormal white blood cells that crowd out healthy ones.
  • The four main types are acute lymphoblastic, acute myeloid, chronic lymphocytic, and chronic myeloid leukemia, each developing at different speeds and affecting different cell types.
  • Symptoms like fatigue, bruising, frequent infections, and bleeding can take weeks or months to appear, or may show up suddenly depending on the type.
  • Diagnosis requires blood tests and usually a bone marrow biopsy to identify which type of leukemia is present.
  • Treatment options include chemotherapy, targeted drugs, radiation, and stem cell transplant, chosen based on the type, stage, and the person's age and overall health.

The four main types of leukemia and how they differ

Leukemia is divided into four main categories based on how fast it grows and which type of white blood cell is affected. Acute lymphoblastic leukemia (ALL) develops quickly in lymphoid cells—a type of white blood cell—and is the most common leukemia in children, though it also occurs in adults. Acute myeloid leukemia (AML) grows rapidly in myeloid cells and is more common in adults over 65, though it can happen at any age.

Chronic lymphocytic leukemia (CLL) develops slowly in lymphoid cells and is usually found in people over 55. Chronic myeloid leukemia (CML) grows slowly in myeloid cells and is often discovered by accident during a routine blood test because people may have no symptoms for months or years. The word "acute" means the cancer grows fast and needs treatment soon; "chronic" means it grows slowly and may not need immediate treatment.

Each type behaves differently, responds to different treatments, and has different outcomes. A person diagnosed with one type will not develop another type—the type is determined by which cells are affected and how the cancer started.

Why leukemia cells crowd out healthy blood cells

Bone marrow normally produces three types of blood cells: red blood cells (which carry oxygen), platelets (which help blood clot), and white blood cells (which fight infection). In leukemia, the marrow becomes filled with abnormal white blood cells that do not work properly and do not die on schedule the way healthy cells do. Because the leukemia cells take up space and use resources, there is less room and less nutrition for normal cells to develop.

This crowding creates a chain of problems. Without enough red blood cells, tissues do not get enough oxygen, causing fatigue and shortness of breath. Without enough platelets, blood does not clot properly, leading to bruising and bleeding. Without enough healthy white blood cells, the immune system cannot fight off infections. The leukemia cells themselves can also damage organs like the liver, spleen, and lymph nodes, making them swell.

Common symptoms and when they appear

Symptoms depend on how many leukemia cells are present and how fast they are multiplying. In acute leukemia, symptoms often appear suddenly over days or weeks: extreme fatigue, shortness of breath, frequent infections, unexplained bruising or bleeding (including nosebleeds or bleeding gums), and bone or joint pain. Some people have a fever or swollen lymph nodes, liver, or spleen.

In chronic leukemia, symptoms may be absent for months or years. When they do appear, they are usually milder: mild fatigue, loss of appetite, weight loss, or a feeling of fullness in the upper left abdomen (where the spleen is). Many people with chronic leukemia discover it only when a blood test done for another reason shows abnormal cell counts.

Symptoms alone cannot diagnose leukemia—they can point to many other conditions. That is why blood tests are the first step when a doctor suspects leukemia.

How leukemia is diagnosed

Diagnosis starts with a complete blood count (CBC), a blood test that measures how many of each type of blood cell you have. In leukemia, the CBC shows too many white blood cells, too few red blood cells, or too few platelets. If the CBC is abnormal, the doctor will order additional blood tests to look at the shape and type of white blood cells and to check for genetic changes that are specific to leukemia.

The next step is usually a bone marrow biopsy. The doctor inserts a needle into the hip bone (or sometimes the breastbone) to remove a small sample of bone marrow. This sample is examined under a microscope and tested for genetic mutations that identify which type of leukemia is present. The biopsy also shows how many leukemia cells are in the marrow and helps doctors plan treatment.

Other tests may include imaging scans (like a CT or ultrasound) to check if leukemia has spread to the liver, spleen, or lymph nodes, and sometimes a spinal tap to see if leukemia cells have reached the fluid around the brain and spinal cord.

Treatment approaches depend on type and stage

Treatment is tailored to the type of leukemia, how advanced it is, the person's age, and their overall health. Chemotherapy—drugs that kill rapidly dividing cells—is the main treatment for acute leukemia and is given in cycles over weeks or months. The goal is to destroy leukemia cells and allow normal bone marrow to recover.

Targeted therapy drugs work differently: they attack specific genetic mutations found in leukemia cells. For example, CML is often treated with tyrosine kinase inhibitors, drugs that block a specific protein that drives the cancer. These drugs can be taken as pills at home and often have fewer side effects than chemotherapy.

Radiation therapy uses high-energy beams to kill leukemia cells and is sometimes used before a stem cell transplant. A stem cell transplant (also called a bone marrow transplant) replaces damaged bone marrow with healthy cells from a donor or from the person's own cells collected and treated earlier. This is often used for people with acute leukemia or CML that is not responding to other treatments.

For chronic leukemia that is not causing symptoms, doctors may recommend "watch and wait"—regular blood tests and monitoring without immediate treatment, since starting treatment early does not always improve outcomes.

What happens after treatment ends

After treatment, people with leukemia need regular follow-up blood tests and bone marrow checks to watch for signs that leukemia is returning. The schedule depends on the type and how well treatment worked. Some people go into remission, meaning leukemia cells are no longer detectable in blood or bone marrow tests, though this does not always mean the cancer is cured.

Remission can last for years or a lifetime, or leukemia can return. If it returns, treatment starts again, often with different drugs. Side effects from chemotherapy or radiation can appear months or years later, so long-term follow-up care includes screening for heart problems, second cancers, and other complications.

Survival rates vary widely by type, age at diagnosis, and how well the leukemia responds to treatment. Acute lymphoblastic leukemia in children now has survival rates above 85% in developed countries, while outcomes for other types depend on many individual factors. A doctor can discuss what to expect based on the specific diagnosis.

Frequently Asked Questions

Is leukemia the same as lymphoma?

No. Leukemia starts in bone marrow and affects blood cells circulating in the bloodstream. Lymphoma is cancer of lymph nodes and lymphoid tissue. Both are blood cancers, but they develop in different places and are treated differently.

Can leukemia run in families?

Most leukemia is not inherited. A few genetic conditions (like Down syndrome) increase the risk, and some families have inherited mutations that raise risk slightly, but most people with leukemia have no family history. Leukemia is not contagious.

What causes leukemia?

The exact cause is usually unknown. Risk factors include exposure to high-dose radiation, certain chemicals like benzene, smoking, previous cancer treatment, and some genetic conditions. Most people with risk factors never develop leukemia, and many people who develop it have no known risk factors.

Can children survive leukemia?

Yes. Survival rates for childhood leukemia have improved dramatically over the past 40 years. About 85% of children with acute lymphoblastic leukemia and about 70% with acute myeloid leukemia survive at least five years. Outcomes depend on the specific type and how well the child responds to treatment.

What is the difference between remission and cure?

Remission means leukemia cells are no longer detectable in blood or bone marrow tests. Cure means the person will never develop leukemia again. Doctors use "remission" because some people's leukemia returns years later, so "cure" cannot always be may provide, even after long periods without signs of disease.