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Understanding leukemia

Types of blood cells

Blood contains three main families of cells: red blood cells carry oxygen, white blood cells protect the body, and platelets stop bleeding. Knowing what each one does helps you understand both the signs of leukemia and your test results.

Last updated: ~6 min read
On a dark navy background, many red disc-shaped cells, a few large whitish white cells and small golden platelets flow inside a translucent glowing blood vessel; a faint DNA helix in the background.
Figure 1.Concept: red blood cells, white blood cells and platelets travel together in the bloodstream.

01Short answer

Blood is made of a liquid part (plasma, about 55%) and cells (about 45%). Red blood cells (erythrocytes) use hemoglobin to carry oxygen from the lungs to the tissues and live for about 120 days. White blood cells (leukocytes) fight infection; there are five main kinds — neutrophils, lymphocytes, monocytes, eosinophils and basophils. Platelets are small cell fragments that help blood to clot. Because leukemia disrupts the normal production of these cells, their numbers in a blood test can change.

02Simple explanation

Picture your blood as a sea: its “water” is plasma, and three kinds of “ships” sail in it. Each one has its own job, and all of them are built in the bone marrow.

Red blood cells are the cargo ships: the protein hemoglobin inside them picks up oxygen in the lungs, delivers it all over the body and brings carbon dioxide back. They are what make blood red. When there are enough of them, you have energy; when there are too few, tiredness, paleness and breathlessness appear — this is called anemia.

White blood cells are the coastguard. There are far fewer of them than red cells, but their work is vital: they find, swallow and destroy germs, make antibodies and keep a “memory” of past infections. When white cells are low or do not work properly, infections can happen often and be severe.

Platelets are the repair crew. When a blood vessel is damaged, they gather at the spot, stick together and help form a clot. When platelets are low, bruising comes easily, tiny red spots may appear on the skin, and the nose or gums may bleed.

In leukemia, the bone marrow starts making abnormal white cells. The total white cell count can then be high, normal or low, but the number of working cells usually falls. That is why doctors look not just at the total, but at each type of cell separately.

03Scientific explanation

Blood makes up about 7–8% of body weight. Most of the cellular part consists of red blood cells; their share of the blood volume is called the hematocrit. Red cells have no nucleus, so they can change shape easily and squeeze through the narrowest capillaries. Their production is controlled mainly by the hormone erythropoietin (EPO), made by the kidneys; a red cell matures in the marrow for about 7 days and survives in the blood for an average of 120 days.

White blood cells make up about 1% of the blood. Neutrophils are the most common type (roughly 40–70% of white cells, depending on the source) and the first line of defence against bacteria and fungi; each neutrophil lives less than a day, so the marrow has to make them constantly. Lymphocytes fall into two main groups: T cells regulate other immune cells and attack infected cells, while B cells make antibodies; NK cells also belong to this family. Monocytes move into tissues and become macrophages, eosinophils take part in defence against parasites and in allergic reactions, and basophils in inflammatory signalling.

Platelets are not whole cells but fragments of large marrow cells called megakaryocytes. When a vessel is injured, they stick, become activated and, together with clotting proteins, build a mesh of fibrin. Platelets live for about 7–10 days.

The myeloid line gives rise to red cells, platelets, neutrophils, monocytes, eosinophils and basophils; the lymphoid line produces lymphocytes. The names of leukemias are based on this split: myeloid leukemias (AML, CML) and lymphoid leukemias (ALL, CLL). A complete blood count (CBC) with differential shows the numbers and proportions of these cells, and a blood smear shows what they look like, including whether blasts are present.

04What happens in the body?

Red blood cells: the oxygen carriers

Every red blood cell is packed with hemoglobin, which binds oxygen in the lungs and releases it in the tissues. In a blood test, the red cell count, hemoglobin and hematocrit are read together. When hemoglobin is low, the doctor looks for the cause of anemia; in leukemia the main cause is that the marrow cannot make enough red cells.

A translucent human torso; bright red cells leave the lungs and spread through the vessels to the whole body, with soft glowing specks representing oxygen.
Figure 2.Red blood cells pick up oxygen in the lungs and carry it all over the body.

White blood cells: five kinds of defender

Neutrophils swallow bacteria, lymphocytes create a targeted immune response and antibodies, monocytes become “clean-up” macrophages in the tissues, and eosinophils and basophils take part in reactions to allergens and parasites. The differential shows the percentage and absolute number of each kind.

Platelets: the repair crew

Platelets stick to a damaged vessel wall and, together with clotting proteins, form a “plug”. When they are low, bruises, petechiae (tiny red spots on the skin) and bleeding can occur. When the count is very low, doctors may recommend a platelet transfusion.

Close-up: a large white cell with a grainy surface engulfs a small germ; nearby, golden platelets gather and stick together on a damaged vessel wall.
Figure 3.White blood cells protect the body, while platelets “patch up” a damaged vessel.

How does leukemia affect these cells?

As leukemia cells multiply in the marrow, production of normal blood cells falls. This leads to three main problems: anemia (tiredness, paleness), a tendency to infections, and bleeding. In some leukemias the blood contains many abnormal white cells, in others very few — the total count on its own neither confirms nor rules out a diagnosis.

05From marrow to lab report

Every blood cell follows the same path: it is born in the marrow, matures, enters the blood and does its job. A complete blood count measures the “end result” of that path.

Stem cell — A marrow stem cell divides and makes new cells.1Stem cellA marrow stem cell divides and makes new cells.
Maturing — The cell matures along the myeloid or lymphoid path.2MaturingThe cell matures along the myeloid or lymphoid path.
Into the blood — Mature cells pass through vessel walls into the bloodstream.3Into the bloodMature cells pass through vessel walls into the bloodstream.
Blood test — The CBC shows their numbers and proportions.4Blood testThe CBC shows their numbers and proportions.
Too few red cellsTiredness, paleness, breathlessness.
Too few working white cellsFrequent or severe infections, fever.
Too few plateletsBruising, tiny red spots, bleeding.

06The main blood cells at a glance

CellMain jobApproximate adult range*What can happen when it is low?
Red blood cellsCarry oxygen and carbon dioxideMen: 4.2–5.7 million/µL; women: 3.8–5.1 million/µLAnemia: tiredness, paleness, breathlessness
HemoglobinThe oxygen-binding protein inside red cellsMen: 13.8–17.2 g/dL; women: 12.1–15.1 g/dLSigns of anemia
White blood cells, totalProtect against infection4,500–11,000/µLTendency to infections
NeutrophilsSwallow bacteria and fungi40–60% of white cellsNeutropenia: risk of bacterial infections
LymphocytesT, B and NK cells: targeted immunity, antibodies20–40% of white cellsTendency to viral and other infections
MonocytesBecome macrophages in tissues; clean-up2–8% of white cellsWeaker defence against some infections
Eosinophils and basophilsReactions to parasites and allergensEosinophils 1–4%, basophils 0.5–1%Usually no symptoms on their own
PlateletsHelp blood to clot150,000–400,000/µLBruising, bleeding; risk rises below 50,000/µL

* Figures are approximate ranges from MedlinePlus; they vary between laboratories and are different in children. Always read your results against your own lab’s ranges and together with your doctor.

07Key terms

Plasma
The liquid part of blood; carries water, salts, proteins and hormones.
Red blood cell (erythrocyte)
A blood cell that carries oxygen.
Hemoglobin
The iron-containing protein in red cells that binds oxygen.
Hematocrit
The percentage of blood volume made up of red cells.
White blood cell (leukocyte)
A blood cell that protects against infection.
Neutrophil
The most common white cell; the first defence against bacteria.
Lymphocyte
T, B and NK cells; the targeted immune response.
Platelet
A small cell fragment that helps blood to clot.
Anemia
A lower-than-normal red cell count or hemoglobin level.
Differential
The percentages and counts of each type of white cell.

08Key points

  • Blood is made of plasma (~55%) and cells (~45%).
  • Red blood cells carry oxygen and live for about 120 days.
  • There are five main types of white cell; neutrophils are the most common and live less than a day.
  • Platelets help blood to clot and live for about 7–10 days.
  • All blood cells are made in the bone marrow, along the myeloid or lymphoid line.
  • Leukemia reduces the production of normal cells, raising the risk of anemia, infection and bleeding.
  • Normal ranges depend on the laboratory; go through your results with your doctor.

09Frequently asked questions

Is the white cell count always high in leukemia?

No. Although the word “leukemia” means “white blood”, at diagnosis some people have a very high white count, some a normal count and some a low one. What matters most is not the number but the type and maturity of the cells. That is why doctors look at a blood smear and examine the bone marrow.

What is the “absolute neutrophil count”?

It is the actual number of neutrophils in the blood rather than their percentage: the total white cell count multiplied by the share of neutrophils. It is a key measure of infection risk. We cover it in detail in “What is neutropenia?”.

One of my results is slightly outside the range — should I worry?

Small deviations are common in healthy people too, and often mean nothing. Your doctor looks at the whole picture: other results, symptoms and changes over time. Try not to interpret results on your own — talk them through with your doctor.

If my hemoglobin is low, should I take iron?

Not every anemia is iron deficiency. In leukemia, anemia usually happens because the marrow is not making enough cells, and iron will not fix that. Check with your hematologist before taking any supplement or medicine.

Why are blood transfusions given?

When the marrow is temporarily making few cells during treatment, doctors can support the body with red cell or platelet transfusions. This keeps you safe while the marrow recovers. When and why they are needed is explained in “Supportive care and transfusions”.

Are the normal ranges different for children?

Yes. In children, and especially in babies, the normal ranges for many blood values differ from adults and change with age. Always read a child’s results against age-appropriate ranges, together with the pediatrician or pediatric hematologist.

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. Which blood cells in my results are outside the normal range, and what does that mean?

  2. At what levels should I call you straight away?

  3. When is a red cell or platelet transfusion needed?

  4. How will my blood counts change during treatment?

My questions

12Sources

  1. American Society of Hematology (ASH). Blood Basics
  2. MedlinePlus (U.S. National Library of Medicine). Complete blood count (CBC)
  3. MedlinePlus (U.S. National Library of Medicine). Blood differential test
  4. MedlinePlus (U.S. National Library of Medicine). Hemoglobin test
  5. MedlinePlus (U.S. National Library of Medicine). Platelet count
  6. MedlinePlus (U.S. National Library of Medicine). Blood
  7. Cleveland Clinic. Platelets: What They Are, Function & Normal Range
  8. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: white blood cell
  9. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: neutrophil
  10. National Heart, Lung, and Blood Institute (NHLBI). Anemia
  11. National Heart, Lung, and Blood Institute (NHLBI). Thrombocytopenia
  12. American Cancer Society. Signs and Symptoms of Acute Myeloid Leukemia (AML)
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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