Leukocyte is another name for white blood cell (WBC). These are the cells in your blood that help your body fight infections and some diseases.

When the number of white cells in your blood is higher than normal, it’s called leukocytosis. This usually happens because you’re sick, but sometimes it’s just a sign that your body is stressed.

Leukocytosis is classified by the type of WBC that’s increased. The five types are:

  • Neutrophilia. This is an increase in WBCs called neutrophils. They’re the most common type of WBCs, accounting for 40 to 60 percent of your WBCs. Neutrophilia is the type of leukocytosis that occurs most often.
  • Lymphocytosis. About 20 to 40 percent of your WBCs are lymphocytes. An increased number of these cells is called lymphocytosis. This type of leukocytosis is very common.
  • Monocytosis. This is the name for a high number of monocytes. This cell type makes up only about 2 to 8 percent of your WBCs. Monocytosis is uncommon.
  • Eosinophilia. This means there are a high number of cells called eosinophils in your blood. These cells make up about 1 to 4 percent of your WBCs. Eosinophilia is also an uncommon type of leukocytosis.
  • Basophilia. This is a high level of WBCs called basophils. There aren’t many of these cells in your blood — only 0.1 to 1 percent of your WBCs. Basophilia is rare.

Each type of leukocytosis tends to be associated with a few conditions:

  • Neutrophilia is associated with infections and inflammation.
  • Lymphocytosis is associated with viral infections and leukemia.
  • Monocytosis is associated with certain infections and cancer.
  • Eosinophilia is associated with allergies and parasites.
  • Basophilia is associated with leukemia.

Leukocytosis itself can cause symptoms. If the number of WNCs is high, it makes your blood so thick that it can’t flow properly. This is a medical emergency that can cause:

  • a stroke
  • problems with your vision
  • breathing problems
  • bleeding from areas covered with mucosa, such as your mouth, stomach, and intestines

This is called hyperviscosity syndrome. It happens with leukemia, but it’s rare.

Other symptoms of leukocytosis are related to the condition causing your high number of WBCs, or sometimes due to effects of the specific type of white blood cell. These may include:

  • fever and pain or other symptoms at the site of an infection
  • fever, easy bruising, weight loss, and night sweats with leukemia and other cancers
  • hives, itchy skin, and rashes from an allergic reaction on your skin
  • breathing problems and wheezing from an allergic reaction in your lungs

You may have no symptoms if your leukocytosis is related to stress or a reaction to a drug.

Causes of leukocytosis can be classified by type of WBC.

Causes of neutrophilia:

  • infections
  • anything that causes long-term inflammation, including injuries and arthritis
  • reaction to some drug such as steroids, lithium, and some inhalers
  • some kinds of leukemia
  • a reaction to emotional or physical stress from things like anxiety, surgery, and exercise
  • having your spleen removed
  • smoking

Causes of lymphocytosis:

  • viral infections
  • whooping cough
  • allergic reactions
  • some kinds of leukemia

Causes of eosinophilia:

  • allergies and allergic reactions, including hay fever and asthma
  • parasite infections
  • some skin diseases
  • lymphoma (cancer associated with the immune system)

Causes of monocytosis:

Causes of basophilia:

  • leukemia or bone marrow cancer (most often)
  • occasionally allergic reactions (occasionally)

Pregnant women usually have higher-than-normal WBC levels. These levels increase gradually, and by the last three months of pregnancy the WBC count is typically between 5,800 and 13,200 per microliter of blood.

The stress of labor and delivery can also increase WBCs. It remains slightly above normal (around 12,700 per microliter of blood) for a while after the baby is born.

Normally you have between 4,000 and 11,000 WBCs per microliter of blood if you aren’t pregnant. Anything higher is considered leukocytosis.

WBC counts between 50,000 and 100,000 per microliter usually mean a very severe infection or cancer somewhere in the body.

A WBC count over 100,000 most often occurs with leukemia or other blood and bone marrow cancer.

There are three tests your doctor might use to help determine why your WBC is higher than normal:

  • Complete blood count (CBC) with differential. This test is almost always done when your WBC count is higher than normal for unknown reasons. For this test, blood drawn from your vein is run through a machine that identifies the percentage of each type of WBC. Knowing which types have higher than normal percentages can help your doctor narrow down the possible causes of your high WBC count.
  • Peripheral blood smear. This test is done when neutrophilia or lymphocytosis is found because your doctor can see if there are too many of the different types of leukocytes. For this test, a thin layer of your blood sample is smeared on a slide. A microscope is then used to look at the cells.
  • Bone marrow biopsy. Your WBCs are made in your bone marrow and then released into your blood. When a high number of certain types of neutrophils are found on your peripheral smear, your doctor may perform this test. Samples of your bone marrow are removed from the center of a bone, usually your hip, with a long needle and examined under a microscope. This test can tell your doctor if there are abnormal cells or a problem with the production or release of cells from your bone marrow.

Treatment of leukocytosis is based on what’s causing it:

  • antibiotics for infection
  • treatment of conditions that cause inflammation
  • antihistamines and inhalers for allergic reactions
  • chemotherapy, radiation, and sometimes a stem cell transplant for leukemia
  • medication changes (if possible) if the cause is a drug reaction
  • treatment of causes of stress and anxiety if they are present

Hyperviscosity syndrome is a medical emergency treated with intravenous fluids, medications, and other methods of quickly getting the WBC count down. This is done to make the blood less thick so it flows normally again.

The best way to prevent leukocytosis is to avoid or reduce the risk of the things that cause it. This includes:

  • maintaining a healthy lifestyle, including good handwashing to avoid an infection
  • staying away from anything you know that might trigger an allergic reaction
  • quitting smoking to avoid smoking-related leukocytosis, and lower your risk of cancer
  • taking medication as directed if you’re being treated for a condition that causes inflammation
  • trying to lower the amount of stress in your life, and getting treated for serious anxiety or emotional problems

Leukocytosis is usually a response to an infection or inflammation, so it’s not a cause for alarm. However, it can be caused by serious diseases such as leukemia and other cancers, so it’s important that your doctor diagnosis the cause of an increased WBC when it’s found. Leukocytosis associated with pregnancy or in response to exercise is normal and nothing to worry about.