White blood cells, also called leukocytes, are the blood cells that help the body fight infection. There are five main kinds, and on a stained blood sample they range from about 6 to 20 micrometers across. A micrometer is a millionth of a meter, or a thousandth of a millimeter. The explorer shows a white blood cell 10 micrometers across, within that range.

Size

A white blood cell's size is its diameter. It is usually measured in a blood smear, a drop of blood spread into a thin layer on a microscope slide and stained. On a smear, the five kinds measure:

• Neutrophils are 12–15 micrometers across and make up 60–70 percent of white blood cells.
• Lymphocytes are 6–9 micrometers across when small and 9–15 when large, and make up 20–30 percent.
• Monocytes are 12–20 micrometers across, the largest kind.
• Eosinophils are 12–15 micrometers across and make up 2–4 percent.
• Basophils are 10–12 micrometers across and make up less than 1 percent.

Cells look larger on a smear than they are in the blood. A 1980 study measured white blood cells in an isotonic solution, a salt solution with the same concentration as body fluids. It found them round, with many folds in their surface, and significantly smaller than on smears.

Size comparisons

• A neutrophil is about 1.5–2 times as wide as a red blood cell, which is 7–8 micrometers across.
• Eight to ten white blood cells 10 micrometers across, lined up side by side, would span the width of a human hair, which is 80–100 micrometers wide.
• The largest monocytes are more than three times as wide as the smallest lymphocytes.
• A microliter of blood, a millionth of a liter, holds 4,500–11,000 white blood cells but about 4–6 million red blood cells, roughly 350–1,300 red cells for every white one.
• If a neutrophil were enlarged to the width of a basketball, about 24 centimeters (9.4 inches), an E. coli bacterium of the kind it destroys would be about 3–4 centimeters (1.3–1.6 inches) long.

What each kind does

Neutrophils destroy bacteria. They gather quickly in tissue that is inflamed, but each one lives only a short time: it stays in the blood for 8–16 hours and survives in the tissues for only a day or two. Eosinophils multiply in allergies and parasite infections and help kill parasites. Basophils release substances that set off inflammation.

Lymphocytes recognize foreign substances and help regulate the body's defenses. Some produce antibodies, molecules that attach to one specific foreign substance. Monocytes spend only about 16 hours in the blood. They then move into the tissues and become macrophages, cells that engulf and digest microbes and dead cells.

Leaving the blood

White blood cells can leave the blood and work in the tissues around it. Lymphocytes routinely leave capillaries, the smallest blood vessels, and wander through the tissues. They return to the blood through the lymphatic system. When tissue becomes inflamed, neutrophils gather there quickly, and lymphocytes arrive somewhat later.

Where the name comes from

White blood cells get their name from their color. When blood is spun in a centrifuge, it separates into layers: clear plasma, a creamy layer of white blood cells, and a red layer of red blood cells.

Sources

• WBC count. MedlinePlus, National Library of Medicine, 2025.
• RBC count. MedlinePlus, National Library of Medicine, 2026.
• MH 033hr Blood Smear. Histology Guide, University of Minnesota.
• Peripheral Blood. Histology Guide, University of Minnesota.
• Blood cells. Histology at SIU, Southern Illinois University School of Medicine, 2024.
• Morphometry of human leukocytes. Schmid-Schönbein et al., Blood, 1980.
• The International System of Units (SI), 9th edition. Bureau International des Poids et Mesures, 2026.
• Just How Small Is "Nano"?. National Nanotechnology Initiative.
• How big is an E. coli cell and what is its mass?. In Cell Biology by the Numbers, 1st edition. Garland Science, 2015.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.