Leukocytosis is a general term for abnormally high levels of white blood cells. This common condition may occur due to individual variations in blood composition but is usually a sign of an underlying health concern.

What Is Leukocytosis?

White blood cells (WBCs), also called leukocytes, are immune system cells that help defend the body from infection and harmful foreign substances. There are several types of WBCs, each with a different function. Normally, about 100 billion WBCs are produced in the bone marrow every day. They circulate through the bloodstream, seeking out and destroying infectious organisms and harmful foreign substances.

Leukocytosis may involve elevated levels of one type of WBC or all types, including:

  • basophils, the least numerous WBCs, perform several roles in the body’s immune response, particularly inflammatory reactions to allergens
  • eosinophils target allergens, cancer cells and parasites
  • lymphocytes (T cells and B cells) are among the immune system’s natural killer cells, which work together to fight invading germs, toxins and cancer
  • monocytes surround and consume foreign material and dead tissue cells, a progress called phagocytosis
  • neutrophils, the most abundant WBC, help fight infection by destroying bacteria and fungi

Types of Leukocytosis

Leukocytosis is classified into five types, depending on which WBC is elevated. Each type is associated with health conditions that may suggest a possible cause, such as:

  • neutrophilia suggests infection, inflammation, metabolic disorders, blood disorders or certain cancers
  • eosinophilia suggests allergic reactions, drug hypersensitivity or parasitic infections
  • monocytosis suggests bacterial infections or autoimmune diseases
  • lymphocytosis suggests viral infections or leukemia
  • basophilia suggests allergies, chronic inflammation or bone marrow disorders

Causes of Leukocytosis

Elevated levels of WBCs may occur for a wide variety of reasons, including disease, infection or other medical conditions. Some common causes of leukocytosis include:

  • allergic reactions
  • autoimmune disorders such as systemic lupus erythematosus
  • bone marrow disorders such as leukemia and other myeloproliferative disorders
  • certain cancers of the blood, bones and organs
  • chronic inflammatory conditions such as rheumatoid arthritis
  • emotional stress
  • genetic conditions such as hereditary neutrophilia
  • infection with bacteria such as tuberculosis, viruses such as Epstein-Barr or parasites
  • medications such as corticosteroids
  • obesity
  • pregnancy or giving birth
  • skin conditions such as dermatitis herpetiformis
  • smoking
  • splenectomy (spleen removal)
  • traumatic injury or surgery

Signs and Symptoms of Leukocytosis

Leukocytosis symptoms depend on the underlying cause and may range from mild to severe. They may include:

  • easy bleeding or bruising
  • fatigue or weakness
  • fever
  • headache or dizziness
  • hives and itching
  • loss of appetite
  • night sweats
  • shortness of breath or wheezing
  • unintended weight loss

How Is Leukocytosis Diagnosed?

In addition to a review of personal and family medical history and a physical examination, leukocytosis diagnosis involves laboratory tests as:

  • complete blood count (CBC)
  • peripheral blood smear, in which a blood sample is examined under a microscope to look for signs of disease or evaluate blood cell abnormalities

Results of these tests will guide the need for additional testing, such as:

  • allergy testing
  • autoimmune disease tests
  • tests for bacterial, viral or parasitic infection
  • tests for signs of inflammation

Depending on the suspected cause, further diagnostic procedures may be needed, such as bone marrow aspiration and biopsy, imaging studies, or molecular genetic testing.

Treatments for Leukocytosis

Treatment for leukocytosis is usually focused on the underlying cause. Options may include:

  • allergy, asthma, infection or inflammation medications
  • chemotherapy or radiation treatments for cancer
  • leukapheresis, a procedure to lower leukocyte counts
  • stem cell transplants for leukemia