Understanding leukemia

What is a cell?

A calm introduction to the body’s smallest living building block: what a cell is made of, how it knows what to do, and why understanding it helps you understand leukemia.

Last updated: 30 September 2026~7 min read

A glowing, semi-transparent round cell on a dark navy background; a violet nucleus in the centre, orange oval mitochondria and small granules around it, with a faint DNA helix in the background. — **Figure 1.**Concept: inside a translucent cell — the nucleus holding DNA at the centre, surrounded by energy-producing mitochondria and tiny protein-making structures.

01Short answer

A cell is the smallest living unit of the body. The human body is made of trillions of cells — skin, muscle, nerve, liver and blood cells among them. Almost every cell has an outer covering (the membrane), a jelly-like interior (the cytoplasm) and tiny “organs” called organelles. Most cells also have a nucleus, which holds DNA — the complete set of instructions. A healthy cell keeps tight control over when it grows, divides, specialises and dies. Leukemia begins when this control breaks down in a blood-forming cell.

02Simple explanation

Imagine a cell as a small but very well-run city. The city has a wall with gates — the cell membrane — which decides what comes in and what goes out. On the wall are “doorbells” called receptors: neighbouring cells and hormones ring them to send the cell messages.

At the centre of the city is a large archive and library — the nucleus. It holds every instruction for building and running the city, written in a long molecule called DNA. Books never leave this library: instead, a copy of the page that is needed is made and sent out to the workshops.

The city’s power stations (mitochondria) turn food into energy, its workshops (ribosomes) assemble proteins by following the instructions, and its recycling centres (lysosomes) take apart worn-out parts and clear away waste. Almost all of the work in a cell is done by proteins.

Nearly every cell in the body carries the same DNA, but each cell reads only the “chapters” it needs. That is why a nerve cell sends signals, a muscle cell contracts and a red blood cell carries oxygen. This process of specialising is called differentiation (maturation).

Cells have a life cycle too: they are born, do their job, divide when needed and, when their time comes, quietly die to make room for new ones. Blood cells are renewed especially quickly. In leukemia, a young blood cell in the bone marrow gets an error in its instructions: it keeps dividing, does not mature and does not die on time. Knowing how a cell normally works makes it much easier to see what that error means.

03Scientific explanation

The cell is the basic structural and functional unit of living things. Human cells are eukaryotic: their DNA is kept in a nucleus surrounded by a membrane. On the outside, the cell is enclosed by the plasma membrane, a double layer of lipids studded with transporter proteins, channels and receptors. Inside is the cytoplasm: the fluid cytosol, the cytoskeleton and the organelles.

The main organelles are the mitochondria, which convert energy from food into a form the cell can use (ATP) and carry a small DNA of their own; the ribosomes, which build proteins; the endoplasmic reticulum and Golgi apparatus, which process and “package” proteins and lipids; and the lysosomes, which break down worn-out components. The nucleus is wrapped in the nuclear envelope and acts as the command centre that tells the cell to grow, mature, divide or die.

Human DNA contains about 3 billion “letters” (base pairs) and is divided into 46 chromosomes. There are roughly 20,000 protein-coding genes. A gene’s information is first copied into RNA (transcription) and then translated into protein on a ribosome (translation). Which genes are read is decided by regulatory proteins and by chemical “tags” on the DNA (epigenetic regulation) — and differentiation depends on exactly this selective reading.

Cells “talk” to each other using signalling molecules. A growth factor binds to its receptor, a chain of proteins inside the cell (for example, kinases) switches each other on in turn, and certain genes are eventually activated in the nucleus. In blood formation, examples of such signals include erythropoietin (EPO), thrombopoietin and granulocyte colony-stimulating factor (G-CSF). Normal tissue keeps a precise balance between cell division, maturation and programmed cell death (apoptosis); cancer, including leukemia, is a disruption of that balance caused by genetic changes.

04What happens in the body?

Membrane and signals

Every cell “listens” to its surroundings. Receptors on the membrane recognise hormones, growth factors and signals from neighbouring cells. When a signal arrives, a kind of relay race starts inside the cell, and the message reaches the nucleus: “divide”, “wait”, “mature” or “end your life”. In some leukemias one link in this relay is stuck in the “on” position — for example, the BCR::ABL1 protein in chronic myeloid leukemia.

The nucleus — the instruction centre

The DNA in the nucleus is packaged into chromosomes. Before dividing, a cell copies all of its DNA precisely, so that each new cell receives a complete set of instructions. Copying errors happen, but the cell has proofreading and repair systems that find and fix them. Errors that are not repaired can become mutations.

Close-up of a translucent nucleus: glowing blue strands of DNA and coiled chromosomes inside, with thin streams of light passing through pores in the nuclear envelope. — **Figure 2.**Inside the nucleus, DNA is packaged into chromosomes; the cell copies the genes it needs and uses those copies to build proteins.

Energy and production

Mitochondria supply the cell with energy, and ribosomes produce proteins. Proteins work as enzymes, transporters, antibodies, signalling molecules and parts of the cell’s framework. For example, hemoglobin in red blood cells is a protein that carries oxygen, and the antibodies made by B lymphocytes are proteins that recognise germs.

Blood cells are cells too

Blood cells are specialised cells made in the bone marrow. A mature red blood cell has no nucleus: it is a flexible little “bag” filled with hemoglobin. White blood cells do have a nucleus, and they fight infection. Platelets are not whole cells at all, but small fragments that break off a large marrow cell called a megakaryocyte.

Red disc-shaped red blood cells, a few pale granular white blood cells and small yellowish platelets floating on a dark background, all softly glowing. — **Figure 3.**Blood cells are cells too: red cells without a nucleus, white cells with a nucleus and tiny platelets travelling in the same bloodstream.

05How does a cell “make decisions”?

A healthy cell’s behaviour is like a chain that starts with a signal and ends with a specific action. In leukemia, one of the links in this chain is broken.

1A signal arrivesA growth factor or hormone binds to a receptor on the membrane.

2The message reaches the nucleusA chain of proteins inside the cell passes the signal on.

3Genes are readCopies of the right genes are made, and proteins are built.

4The cell actsIt divides, matures, works — or dies on time.

Division when neededNew cells are made only when the body needs them.

Proper maturationEach cell reaches the form in which it can do its job.

Death on timeOld and damaged cells make room for new ones.

06The main parts of a cell

Part	What it does	Simple comparison	Link to leukemia
Cell membrane	Encloses the cell and controls what passes through	City wall and gates	CD markers on its surface help identify the type of leukemia
Receptors	Recognise signals from outside	Doorbell	Some targeted drugs act on receptors or on the signalling chain
Cytoplasm	Jelly-like interior where the organelles sit	City streets	How a cell looks under the microscope (for example, its granules) helps diagnosis
Nucleus	Stores DNA and directs the cell’s work	Archive and library	The large nucleus of blasts is one of the features seen under the microscope
DNA and chromosomes	Hold all the instructions	Books and pages	Leukemia starts with acquired changes in DNA
Mitochondria	Produce energy from food	Power station	Some drugs act on the cell’s energy and survival pathways
Ribosomes	Build proteins	Workshop	Abnormal genes produce abnormal proteins

* This table is a simplified overview. CD markers and targeted treatments are covered in more detail in separate topics.

07Key terms

Cell: The smallest structural and functional unit of a living organism.
Membrane: The thin covering around a cell that controls what goes in and out.
Cytoplasm: The jelly-like fluid and organelles inside the membrane but outside the nucleus.
Organelle: A tiny “organ” inside a cell that does a particular job (for example, a mitochondrion).
Nucleus: The centre where DNA is stored and the cell’s work is directed.
DNA: The long molecule that carries all of a cell’s instructions.
Protein: A molecule built from a gene’s instructions that does most of the work in a cell.
Receptor: A protein on or inside a cell that recognises a signalling molecule and responds to it.
Differentiation: The maturing of a cell into a specialised form that does a particular job.
Apoptosis: Controlled, “planned” cell death; the body’s way of removing damaged and old cells.

08Key points

A cell is the smallest living unit of the body; the human body is made of trillions of cells.
The membrane protects the cell and receives signals; the nucleus stores the instructions (DNA).
Genes are instructions for making proteins, and proteins do most of the work in a cell.
Cells with the same DNA specialise in different jobs by reading different genes.
A healthy cell divides only when needed, matures, and dies on time.
Blood cells are made in the bone marrow and are renewed very quickly.
Leukemia begins when the balance between division, maturation and death is lost in a blood-forming cell.
Knowing these basics makes your doctor’s explanations and your test results easier to follow.

09Frequently asked questions

How many cells are in the body?

The exact number varies from person to person, but reputable sources describe the human body as being made of trillions of cells. A very large share of them are blood cells, especially red blood cells.

Do all cells have the same DNA?

Almost: most cells in the body carry the same set of DNA. The difference is that each type of cell switches different genes on or off. There are exceptions — for example, mature red blood cells have no nucleus and therefore no nuclear DNA. Leukemia cells carry additional DNA changes acquired later in life.

Why do cells have to die? Isn’t that bad?

No — programmed cell death (apoptosis) is essential for health. It clears away old, unneeded or DNA-damaged cells and keeps tissues in balance. One of the problems with leukemia cells is precisely that they do not die when they should.

How is a leukemia cell different from a normal cell?

A leukemia cell comes from a normal blood cell, but its DNA has changed. Because of these changes it no longer follows signals: it divides too much, often fails to mature and lives longer than normal. Doctors detect these differences using the microscope, flow cytometry and genetic tests.

Why do I need to know about cells?

During treatment you will often hear words like “blast”, “CD marker”, “mutation” and “cell cycle”. Understanding how a cell works makes these words easier to follow, helps you ask your doctor more precise questions and can make things feel less frightening. Only your care team can explain what applies to your own situation.

Can foods or supplements “strengthen” cells and cure leukemia?

A balanced diet matters for your overall health, but no food, herb or supplement “repairs” leukemia cells or replaces treatment. Some supplements can interact with medicines, so discuss any supplement with your doctor first.

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.

How are leukemia cells different from healthy cells?
How will treatment affect my healthy cells?
Which cells in my body recover fastest after treatment?
Can you recommend a reliable source that explains what is happening at the cell level?

My questions

12Sources

MedlinePlus Genetics (U.S. National Library of Medicine). What is a cell?
MedlinePlus Genetics (U.S. National Library of Medicine). What are proteins and what do they do?
MedlinePlus Genetics (U.S. National Library of Medicine). How do cells divide?
National Human Genome Research Institute (NHGRI). Talking Glossary of Genomic and Genetic Terms: Cell
National Human Genome Research Institute (NHGRI). Talking Glossary: Nucleus
National Human Genome Research Institute (NHGRI). Talking Glossary: Mitochondria
National Human Genome Research Institute (NHGRI). Talking Glossary: Cell Membrane (Plasma Membrane)
National Human Genome Research Institute (NHGRI). Talking Glossary: Ribosome
National Cancer Institute (NCI). What Is Cancer?
National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: cell
American Society of Hematology (ASH). Blood Basics

Medical disclaimerThis information is for educational purposes only and does not replace medical advice.

Last updated: 30 September 2026← Back to the list

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