White blood cells circulate around the blood and help the immune system fight off infections.

Stem cells in the bone marrow are responsible for producing white blood cells. The bone marrow then stores an estimated 80–90% of white blood cells.

When an infection or inflammatory condition occurs, the body releases white blood cells to help fight the infection.

In this article, learn more about white blood cells, including the types and their functions.

Types and function

Health professionals have identified three main categories of white blood cell: granulocytes, lymphocytes, and monocytes. The sections below discuss these in more detail.

Granulocytes
Granulocytes are white blood cells that have small granules containing proteins. There are three types of granulocyte cells:

Basophils: These represent less than 1% of white blood cells in the body and are typically present in increased numbers after an allergic reaction.
Eosinophils: These are responsible for responding to infections that parasites cause. They also play a role in the general immune response, as well as the inflammatory response, in the body.
Neutrophils: These represent the majority of white blood cells in the body. They act as scavengers, helping surround and destroy bacteria and fungi that may be present in the body.
Lymphocytes
These white blood cells include the following:

B cells: Also known as B-lymphocytes, these cells produce antibodies to help the immune system mount a response to infection.
T cells: Also known as T-lymphocytes, these white blood cells help recognize and remove infection-causing cells.
Natural killer cells: These cells are responsible for attacking and killing viral cells, as well as cancer cells.
Monocytes
Monocytes are white blood cells that make up around 2–8% of the total white blood cell count in the body. These are present when the body fights off chronic infections.

They target and destroy cells that cause infections.

Normal ranges
According to an article in American Family Physician, the normal range (per cubic millimeter) of white blood cells based on age are:

Age Normal range
Newborn infant 13,000–38,000
2-week-old infant 5,000–20,000
Adult 4,500–11,000
The normal range for a pregnant women in the 3rd trimester is 5,800–13,200 per cubic millimeter.

High white blood cell count
If a person’s body is producing more white blood cells than it should be, doctors call this leukocytosis.

A high white blood cell count may indicate the following medical conditions:

allergic responses, such as due to an asthma attack
those that may cause cells to die, such as burns, heart attack, and trauma
inflammatory conditions, such as rheumatoid arthritis, inflammatory bowel disease, or vasculitis
infections, such as with bacteria, viruses, fungi, or parasites
leukemia
Surgical procedures that cause cells to die can also cause a high white blood cell count.

Low white blood cell count
If a person’s body is producing fewer white blood cells than it should be, doctors call this leukopenia.

Conditions that can cause leukopenia include:

autoimmune conditions such as lupus and HIV
bone marrow damage, such as from chemotherapy, radiation therapy, or exposure to toxins
bone marrow disorders
leukemia
lymphoma
sepsis, which is a severe type of infection
vitamin B-12 deficiencies
Doctors may continually monitor white blood cells to determine if the body is mounting an immune response to an infection.

White blood cell test
During a physical examination, a doctor may perform a white blood cell count (WBC) using a blood test. They may order a WBC to test for, or rule out, other conditions that may affect white blood cells.

Although a blood sample is the most common approach to testing for white blood cells, a doctor can also test other body fluids, such as cerebrospinal fluid, for the presence of white blood cells.

A doctor may order a WBC to:

test for allergies
test for infection
test for leukemia
monitor the progression of certain conditions
monitor the effectiveness of some treatments, such as bone marrow transplants
Conditions affecting the white blood cells
The following are conditions that may impact how many white blood cells a person has in their body.

Aplastic anemia
This is a condition wherein a person’s body destroys stem cells in the bone marrow.

Stem cells are responsible for creating new white blood cells, red blood cells, and platelets.

Evans syndrome
This is an autoimmune condition wherein the body’s immune system destroys healthy cells, including red and white blood cells.

HIV
HIV can decrease the amount of white blood cells called CD4 T cells. When a person’s T cell count drops below 200, a doctor might diagnose AIDS.

Leukemia
Leukemia is a type of cancer that affects the blood and bone marrow. Leukemia occurs when white blood cells rapidly produce and are not able to fight infections.

Primary myelofibrosis
This condition causes a person’s body to overproduce some types of blood cells. It causes scarring in a person’s bone marrow.

How to raise or lower white blood cell count
Whether or not a person needs to alter their white blood cell count will depend on the diagnosis.

If they have a medical condition that affects the number of white blood cells in their body, they should talk to a doctor about the goals for their white blood cell count, depending on their current treatment plan.

A person can lower their white blood cell count by taking medications such as hydroxyurea or undergoing leukapheresis, which is a procedure that uses a machine to filter the blood.

If a person’s white blood cell count is low due to cancer treatments such as chemotherapy, a doctor may recommend avoiding foods that contain bacteria. This may help prevent infections.

A person can also take colony-stimulating factors. These may help prevent infection and increase the number of white blood cells in the body.

Summary
White blood cells are an important part of the body’s immune system response. There are different types of white blood cell, and each has a specific function in the body.

Certain conditions can affect the number of white blood cells in the body, causing them to be too high or too low.

If necessary, a person can take medication to alter their white blood cell count.