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Understanding leukemia

What is leukemia?

A calm, step-by-step introduction: where blood is made, what goes wrong in leukemia, and why it causes the symptoms it does.

Last updated: ~4 min read
A glass-like translucent long bone on a dark navy background; the red marrow inside is packed with round red blood cells, among them a few glowing teal stem cells, pale white blood cells and tiny platelets.
Figure 1.Concept: healthy red bone marrow inside a translucent bone. A few stem cells (teal) give rise to red cells, white cells and platelets; mature cells then move into the blood.

01Short answer

Leukemia is a group of cancers of the blood-forming cells. It usually starts in the bone marrow — the soft tissue inside bones where blood cells are made. A change in a cell’s DNA makes it multiply out of control and stop maturing properly. These abnormal cells build up and crowd out healthy blood cells, which can lead to anemia, infections and bleeding. Leukemia is not one disease: there are several types, and each is treated differently.

02Simple explanation

Imagine the bone marrow as a busy “blood factory” inside your bones. Every day it makes billions of new cells: red blood cells that carry oxygen, white blood cells that fight infection, and platelets that help blood clot.

In leukemia, one young blood cell gets a “broken instruction” in its DNA. It starts copying itself again and again, and its copies do not grow up into useful, working cells. Over time these copies fill the factory, leaving less room and fewer resources for healthy cells.

That is why a person may feel tired (too few red cells), catch infections more easily (too few working white cells), or bruise and bleed easily (too few platelets).

Leukemia is not contagious, and in most people it is not inherited. It is usually nobody’s fault.

03Scientific explanation

Leukemias are malignant neoplasms of hematopoietic (blood-forming) stem or progenitor cells. Acquired genetic changes — point mutations, chromosomal translocations, gene fusions, or changes in chromosome number — disturb the normal balance between self-renewal, proliferation, differentiation (maturation) and programmed cell death (apoptosis).

The result is a clone: a population of cells descended from a single altered cell. In acute leukemias the clone is made of immature cells called blasts, whose development is blocked. In chronic leukemias the cells can mature further, but they accumulate in excess.

Leukemias are classified by the cell lineage involved — myeloid (the line that gives rise to red cells, platelets, granulocytes and monocytes) or lymphoid (B, T and NK cells) — and by how quickly they progress. Modern classifications, such as the 5th edition of the WHO Classification of Haematolymphoid Tumours (2022), also rely heavily on genetic features, for example the BCR::ABL1 fusion in CML or PML::RARA in acute promyelocytic leukemia.

04What happens in the body?

At the level of the cell

Normally a cell divides only when it receives the right signals, matures into a specialised cell and eventually dies to make room for new ones. The instructions for all of this are written in DNA. When certain genes that control growth and maturation are damaged, the cell can ignore “stop” signals, divide too often, fail to mature, or survive longer than it should.

In the bone marrow

Blood-forming stem cells in the marrow normally produce a balanced mix of blood cells. Leukemia cells multiply, take up space and resources, and disturb the marrow environment. Production of normal red cells, white cells and platelets falls — this is called bone marrow failure.

Close-up of bone marrow: a large, tightly packed cluster of violet blast cells fills the centre; a few red blood cells, pale white blood cells and teal cells are pushed out to the edges.
Figure 2.Mechanism: abnormal leukemic blasts (violet) multiply quickly in the bone marrow and crowd normal cells out to the edges.

In the blood and organs

Leukemia cells often spill into the bloodstream, so the white cell count may be high — but it can also be normal or low. A high count does not mean better protection, because leukemia cells do not fight infection properly. In some types, leukemia cells also collect in the lymph nodes, spleen, liver or, less often, the central nervous system.

A translucent blood vessel running diagonally across a dark blue background, carrying many violet blast cells, only a few red blood cells and some tiny platelets.
Figure 3.Result: the bloodstream carries many immature white cells (blasts) and fewer red cells and platelets. This is why anemia, infections and bleeding can occur.

05The mechanism, step by step

The core mechanism of leukemia can be shown in four steps and three consequences.

Blood stem cell — A normal blood-forming cell in the bone marrow.1Blood stem cellA normal blood-forming cell in the bone marrow.
DNA change — An acquired change in genes that control growth and maturation.2DNA changeAn acquired change in genes that control growth and maturation.
Uncontrolled blast growth — Abnormal cells multiply but do not mature.3Uncontrolled blast growthAbnormal cells multiply but do not mature.
Marrow crowded out — Less room and support for healthy blood production.4Marrow crowded outLess room and support for healthy blood production.
AnemiaTiredness, pale skin, shortness of breath — too few red cells.
InfectionsFever, frequent infections — too few working white cells.
BleedingBruising, nosebleeds, tiny red spots — too few platelets.

06Main types

TypeFull nameAcute / chronicCell lineTypical age
AMLAcute myeloid leukemiaAcuteMyeloidMostly older adults; can occur at any age
ALLAcute lymphoblastic leukemiaAcuteLymphoidThe most common leukemia in children; also occurs in adults
CMLChronic myeloid leukemiaChronicMyeloidMainly middle-aged and older adults; rare in children
CLLChronic lymphocytic leukemiaChronicLymphoid (B cells)Older adults; very rare in young people
APLAcute promyelocytic leukemia — a subtype of AMLAcuteMyeloidMostly adults, often younger than in other types of AML
HCLHairy cell leukemia — a rare typeChronicLymphoid (B cells)Middle-aged and older adults; more common in men

* Ages are general patterns, not rules — any type can occur outside these ranges. APL needs urgent treatment because of the risk of bleeding, and when treated promptly it often responds very well. Other rare types exist (for example CMML and JMML).

07Key terms

Bone marrow
The soft, spongy tissue inside bones where blood cells are made.
Blood-forming (hematopoietic) stem cell
The “parent” cell that gives rise to every type of blood cell.
Blast
A very young, immature blood cell. A high share of blasts in the marrow or blood is a sign of acute leukemia.
Myeloid / lymphoid
The two main “families” (lineages) of blood cells.
Acute / chronic
Fast-growing leukemia of immature cells / usually slower-growing leukemia of more mature cells.
Anemia
A shortage of red blood cells or hemoglobin.
Neutropenia
Low neutrophils — the white cells that respond first to bacterial infections.
Thrombocytopenia
Low platelets.
Remission
After treatment, no signs of leukemia can be found with standard tests.

08Key points

  • Leukemia is a cancer of blood-forming cells that usually begins in the bone marrow.
  • It is caused by acquired DNA changes in a single cell, which then multiplies into a clone.
  • The main problem is not only “too many abnormal cells” but also “too few healthy cells”.
  • The four main types are AML, ALL, CML and CLL; APL and HCL are important subtypes / rare forms.
  • An exact diagnosis needs blood tests, a bone marrow examination and genetic tests.
  • Leukemia is not contagious and is usually not inherited.
  • Treatment depends on the exact type and its genetic features — your care team will explain what applies to you.

09Frequently asked questions

Is leukemia contagious?

No. Leukemia cannot pass from one person to another through touch, the air, shared dishes or living together.

Is leukemia inherited?

Most leukemias result from DNA changes acquired during a person’s life in blood-forming cells, and they are not passed on to children. A few inherited conditions (for example Down syndrome and some rare inherited syndromes) increase the risk. If you have questions about family risk, ask your doctor or a genetic counsellor.

Did I cause my leukemia?

In most cases the cause of the DNA changes is unknown. Some known risk factors (previous chemotherapy or radiation therapy, exposure to benzene, smoking) raise the risk of certain types, but most people with leukemia have none of them. It is usually not the result of anything a person did or didn’t do.

Does a high white blood cell count always mean leukemia?

No. White cells can rise with infections, inflammation, stress, some medicines and many other conditions. In leukemia the count can be high, normal or low. Only a doctor, using further tests, can make the diagnosis.

Can leukemia be treated?

Yes. Many people with leukemia are treated successfully, and some types have seen major advances in recent decades. Outcomes depend strongly on the type, its genetic features, age and general health, so the most reliable information about your situation comes from your care team. No food, herb or supplement can cure leukemia or replace treatment; always tell your team about any supplements you take, because some can interact with treatment.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Exactly which type of leukemia do I have?

  2. How quickly does this type usually progress?

  3. Which tests confirmed the diagnosis, and which results are we still waiting for?

  4. What is the most important next step for me right now?

My questions

12Sources

  1. National Cancer Institute (NCI). Leukemia — Patient Version
  2. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: leukemia
  3. National Cancer Institute (NCI). Adult Acute Myeloid Leukemia Treatment (PDQ®) — Patient Version
  4. National Cancer Institute (NCI). Hairy Cell Leukemia Treatment (PDQ®) — Patient Version
  5. American Cancer Society. Leukemia
  6. American Cancer Society. What Is Acute Myeloid Leukemia (AML)?
  7. American Cancer Society. What Is Chronic Myeloid Leukemia (CML)?
  8. WHO / IARC. WHO Classification of Tumours, 5th edition — Haematolymphoid Tumours
  9. Leukemia (journal), 2022. Khoury JD et al. The 5th edition of the WHO Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms
  10. Leukemia (journal), 2022. Alaggio R et al. The 5th edition of the WHO Classification of Haematolymphoid Tumours: Lymphoid Neoplasms
  11. NHS (UK). Acute myeloid leukaemia
  12. NHS (UK). Acute lymphoblastic leukaemia
  13. NHS (UK). Chronic lymphocytic leukaemia
  14. NHS (UK). Chronic myeloid leukaemia
  15. Blood Cancer United (formerly The Leukemia & Lymphoma Society). Leukemia
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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