What Is Leukocytosis?

Written by Lisa Guthrie
Medically Reviewed by Lalitha Kadali, MBBS, MD on February 13, 2026

Leukocytosis is when you have a higher-than-normal number of white blood cells. White blood cells, also called leukocytes, are important for making sure your body stays healthy. They protect it from outside substances and fight infection. Leukocytes usually rise in number when they are fighting infection, but they can also increase due to some other conditions.

What qualifies as a normal white blood count isn't the same for everyone. A normal count for adults is from 4,500 to 11,000 per microliter of blood. Newborns have a much higher number, as high as 38,000. This count goes down as the child grows, eventually falling to adult levels. Pregnant women have slightly higher counts. ‌

Often, high white blood cell counts mean you're fighting an infection or inflammation. Sometimes, it can mean a more serious problem, such as leukemia.

Although leukocytes (white blood cells) are very important, they make up only about 1% of your blood. Like other blood cells, they are made in the bone marrow. They are being created all the time because most have a very short lifespan. Some live for less than a day.

There are five major types of white blood cells:‌

When your body is under attack, the white blood cells rush to the problem area. Different types of white blood cells have different roles in fighting invaders:

  • Neutrophils fight bacteria.
  • Eosinophils fight parasites.
  • Monocytes fight foreign organisms and also deal with cell waste.
  • Lymphocytes make up the immune system.
  • The role of basophils isn't clear, but they are also involved in the immune response.

Most often, doctors use a complete blood count (CBC) to check for leukocytosis. A CBC can be part of a routine physical, or your doctor might use it to help diagnose a specific illness. Another test, called a white blood cell differential or "diff," is sometimes done at the same time. It shows counts for the five main types of white blood cells.

You can have a high count of all five types of cells, a few types, or just one type — usually lymphocytes or neutrophils. A high count of these cells is sometimes linked to blood cancers.

Lymphocytes are an important part of the body's defense system. They protect against bacteria, viruses, parasites, and fungal growth. Lymphocytic leukocytosis is a high count of these cells. Lymphocytes come in three types: B cells, T cells, and natural killer cells. By checking which type is higher in number, doctors can find out the exact cause of the high count.

‌The most common cause of lymphocytic leukocytosis is infection with a virus. Other causes include:‌

  • Tuberculosis, a bacterial infection of the lungs
  • Graves’ disease, a disorder of the thyroid gland
  • Crohn’s disease, an inflammatory bowel disease

Two blood cancers, lymphoma and lymphocytic leukemia, are associated with high numbers of lymphocytes in the blood.

Neutrophils, like lymphocytes, protect against bacteria, viruses, parasites, and fungal growth. They also play a role in healing injuries. Myeloid leukemias can cause high neutrophil counts. If doctors find a high number of immature neutrophils in the blood, they may suspect leukemia. Doctors usually diagnose leukemia by analyzing a bone marrow sample.‌‌

A serious condition can result if a person has leukemia and the number of immature neutrophils in the blood goes too high. This rare condition is called hyperviscosity syndrome. It can happen when the blood becomes too thick. It can cause a stroke or breathing problems that could lead to death. Doctors treat this syndrome by adding fluid to the blood and using drugs to reduce the neutrophils in the blood.

Disorders of the white blood cells can be malignant or nonmalignant. That means cancerous or not cancerous. The main non-malignant cause of leukocytosis is infection. A high white blood count most often signals an infection in the body, especially when there are also other symptoms. ‌

Other conditions that can cause leukocytosis include:

‌Leukocytosis can also signal malignant disorders, especially leukemias or other blood cancers. Doctors look at several factors when considering this diagnosis.‌

Symptoms 

Fever, night sweats, and weight loss are three symptoms that suggest a malignant cause of leukocytosis. Doctors also look for evidence of bleeding or easy bruising. Fatigue may be present.

Physical examination 

Doctors check for swollen lymph nodes, an enlarged liver, or an enlarged spleen. They also look for petechiae, which are pinpoint spots caused by bleeding under the skin.‌

Lab results

Blood cancers are the typical cause of white blood counts over 30,000 per microliter of blood. Some of the cells may be immature. Other counts, such as red blood cells and platelets, may be low.

Leukocytosis is when you have a higher-than-normal number of white blood cells. These cells play an important part in keeping you healthy. When your body comes under attack, the white blood cells rush to the area where the problem is. So higher numbers usually mean you're fighting an infection or dealing with inflammation. But sometimes, it can mean a serious condition, such as leukemia.

What symptoms are linked to leukocytosis?

Some leukocytosis symptoms can point to an infection or inflammation. Others could be signs of a more serious condition. Symptoms include: 

  • Trouble breathing
  • Fever
  • Pain
  • Weight loss you don't expect
  • Fatigue (extreme tiredness)
  • Night sweats
  • Rash
  • Wheezing

What treatments are available for leukocytosis?

The kind of treatment you get for leukocytosis depends on what caused it. If you have an allergic reaction, your doctor may give you antihistamines. If an infection causes a high white blood cell count, you might get antibiotics. If you have inflammation, your provider might give you anti-inflammatories.

Is leukocytosis a UTI?

No, but you can have high levels of white blood cells in your urine. That's a condition called pyuria. It's common and often happens along with a urinary tract infection (UTI). 

Add WebMD as a Preferred Source to see more of our trusted content across Google Search, AI Overviews, and AI Mode