What Are White Blood Cells Called? Explained

White blood cells are also called leukocytes. That is their formal medical name, and it comes from the Greek words for “white” and “cell.” Leukocytes are one of the main components of blood, alongside red blood cells, platelets, and plasma.

Most people know them simply as white blood cells. But you will see the term leukocytes on lab reports, in medical articles, and in conversations with doctors. The two names mean the same thing. The number on your lab report — usually labeled “WBC” — is your leukocyte count.

What Are White Blood Cells Called and Why Do They Have Two Names?

Both names describe the same cells. “White blood cell” is the plain-English description. “Leukocyte” is the scientific and clinical term.

The name comes from how these cells look under a microscope. When a sample of blood is spun in a centrifuge, it separates into layers. Red blood cells settle at the bottom. A thin, whitish layer sits on top of them. That pale layer is packed with immune cells — hence “white” blood cells.

They are not actually white in the way milk or snow is white. They are mostly colorless. The name stuck because of that pale buffy layer in a separated blood sample.

Medical labs, textbooks, and research papers use “leukocyte” because it is precise and internationally consistent. Your doctor might say either one. A lab report will almost always say “WBC” or “leukocytes.”

What Are the Five Types of White Blood Cells?

Leukocytes are not one cell type. They are a family of five main types, and each does a different job. They fall into two broad groups: granulocytes and agranulocytes. The difference is whether the cell contains visible granules when stained and viewed under a microscope.

Knowing the five types matters because a blood test called a differential count measures how many of each you have. That breakdown often tells a doctor more than the total white blood cell count alone.

• Neutrophils — the most abundant type, making up roughly 55% to 70% of circulating white blood cells. They are the first responders to bacterial infection and are the body’s main defense against bacteria and fungi.
• Lymphocytes — about 20% to 40% of the total. This group includes T cells, B cells, and natural killer cells. They handle viruses, produce antibodies, and carry immune memory.
• Monocytes — roughly 2% to 8%. They circulate in blood, then move into tissues and become macrophages that engulf debris and microbes.
• Eosinophils — about 1% to 4%. They are involved in allergic reactions and in defending against parasites.
• Basophils — the rarest, usually under 1%. They release histamine and play a role in allergic responses.

These percentage ranges are standard reference values used in clinical labs. Individual labs may set slightly different ranges, so your report’s own reference range is the one that matters.

Neutrophils and lymphocytes together make up the large majority of your white blood cells. That is why a shift in either one tends to move the total count noticeably.

What Does a White Blood Cell Count Actually Measure?

A white blood cell count measures how many leukocytes are circulating in a set volume of blood. In the US, it is reported as cells per microliter (mcL).

A typical reference range for adults is about 4,000 to 11,000 white blood cells per microliter. This range is well established in clinical practice, though exact limits vary slightly between laboratories.

It is worth understanding what this number does and does not tell you. A total count is a rough signal, not a diagnosis. A normal count does not rule out illness, and an abnormal count does not confirm one. Doctors interpret the number alongside symptoms, the differential count, and other tests.

Several things can shift your count without any disease being present. Strenuous exercise, emotional stress, pregnancy, smoking, and certain medications can all affect it. A single reading is a snapshot. Trends over time often carry more meaning than one result.

What Causes a High White Blood Cell Count?

A high white blood cell count is called leukocytosis. The most common cause is the body responding to an infection.

When bacteria invade, neutrophils multiply and rush to the site. When a virus is involved, lymphocytes often rise instead. The pattern of which cell type increases can help a doctor narrow down what is going on, though it rarely identifies the exact cause on its own.

Other causes of a high count include:

• Inflammation from injury, surgery, or inflammatory conditions
• Physical or emotional stress
• Certain medications, including some steroids
• Smoking
• Allergic reactions, which can raise eosinophils
• Bone marrow disorders, including some cancers of the blood

A high count from infection or stress usually returns toward normal once the trigger resolves. A persistently high count with no clear cause deserves medical evaluation.

What Causes a Low White Blood Cell Count?

A low white blood cell count is called leukopenia. It means the body has fewer infection-fighting cells than usual, which can raise the risk of getting sick.

Common causes include viral infections that temporarily suppress the bone marrow, certain medications, autoimmune conditions, and problems with the bone marrow itself. Chemotherapy is a well-known cause because it affects rapidly dividing cells, including those that produce white blood cells.

When the count drops very low, the main concern is infection risk. Doctors monitor these levels closely in people receiving treatments that suppress the bone marrow. A low count on its own does not always mean something is wrong — some people naturally run lower than average.

The neutrophil count specifically matters a great deal here. Neutrophils are the frontline defense against bacteria, so a very low neutrophil count is a more direct signal of infection risk than a low total count alone.

How Do White Blood Cells Protect the Body?

Leukocytes defend the body in several distinct ways, and the type of cell determines the method.

Neutrophils and macrophages work by engulfing. They surround a microbe, pull it inside, and break it down. This process is called phagocytosis. Neutrophils are fast and numerous; macrophages are slower but larger and can keep working for longer.

Lymphocytes work differently. B cells produce antibodies — proteins that lock onto specific targets and mark them for destruction. T cells can directly kill infected cells or coordinate the wider immune response. Some lymphocytes stay behind after an infection as memory cells, which is part of why you usually do not get the same illness twice.

Eosinophils and basophils handle more specialized jobs. Eosinophils target parasites and help drive allergic responses. Basophils release histamine, which is part of why allergic reactions cause swelling and itching.

One detail people often miss: most white blood cells do not stay in the bloodstream. Blood is mainly a transport route. The real work happens in tissues, lymph nodes, and organs. A blood count measures the cells in transit, which is a useful but incomplete picture of what the immune system is doing.

When Should You Pay Attention to Your White Blood Cell Count?

Pay attention when a result falls clearly outside the reference range, or when it changes over time without an obvious reason.

An abnormal count by itself is not a diagnosis. It is a signal that prompts further investigation. Your doctor will look at the differential count, your symptoms, your history, and other lab results together.

If you have a count that is high or low and you also have symptoms like recurring infections, unexplained fevers, easy bruising, or persistent fatigue, that combination is worth a prompt medical conversation. Symptoms plus an abnormal result carry more weight than the number alone.

Do not try to interpret a single value in isolation. Reference ranges vary between labs, and a result just outside the range is often not meaningful. Bring your questions to the clinician who ordered the test — they have the full context that a number on a page does not.