A White Blood Cell Count (WBC) is a common blood test that measures how many white blood cells (also called leukocytes) are present in a sample of your blood. White blood cells are a key part of the immune system. They help your body fight infection, respond to inflammation, and react to allergies, stress, and certain medicines.

The WBC test is usually reported as the number of white blood cells per microliter (µL) of blood. It is often included in a Complete Blood Count (CBC) and may be paired with a WBC differential, which breaks the total into types of white blood cells (such as neutrophils and lymphocytes). The total WBC number gives a big-picture view of immune activity.

What the results can mean (in plain language):

High WBC (leukocytosis) often means your body is reacting to something—most commonly an infection (especially bacterial), inflammation, physical or emotional stress, smoking, or steroid medications. It can also be seen with some bone marrow conditions and blood cancers. A mildly elevated WBC may be temporary, while a very high or persistent elevation usually needs follow-up.

Low WBC (leukopenia) may mean your body is not making enough white blood cells, or that white blood cells are being used up or destroyed. This can happen with some viral infections, autoimmune conditions, vitamin deficiencies, bone marrow problems, or from treatments such as chemotherapy or radiation. A low WBC can increase the risk of getting infections, especially if neutrophils are low.

Because “normal” ranges can vary by laboratory, age, pregnancy status, and medical context, your healthcare provider interprets your WBC test along with your symptoms, medical history, and other lab results. If an abnormal WBC count is found, doctors may repeat the test, order a differential, or look for the underlying cause.

A WBC test is often ordered as part of routine health screening or to evaluate symptoms that may suggest infection, inflammation, or another condition affecting the immune system. It helps clinicians with disease diagnosis and monitoring by showing whether your body appears to be mounting an immune response or has reduced ability to fight infection.

Your clinician may order a WBC test if you have symptoms such as:

• Fever or chills
• Fatigue, weakness, or feeling generally unwell
• Persistent cough, shortness of breath, or chest symptoms
• Sore throat, sinus pain, ear pain, or signs of respiratory infection
• Abdominal pain, nausea/vomiting, diarrhea, or urinary symptoms (burning, frequency)
• Redness, swelling, warmth, pus, or slow-healing wounds
• Unexplained weight loss, night sweats, or ongoing swollen lymph nodes
• Easy bruising/bleeding or recurrent infections

It may also be ordered to:

• Check for infection after surgery, injury, or during hospitalization
• Evaluate or monitor chronic inflammation (for example, autoimmune disease flare-ups)
• Monitor the effects of medications that can change white blood cells (e.g., steroids, some psychiatric medicines, immunosuppressants)
• Monitor patients receiving chemotherapy, radiation therapy, or other treatments that affect bone marrow
• Help investigate abnormal findings on other tests (such as anemia or platelet changes) as part of a broader CBC review

In many cases, WBC changes are not specific to one single disease—rather, they point to the need for further evaluation, especially when symptoms are present or when results are significantly outside the expected range.

• Acute and chronic infections (e.g., bacterial infections, viral illnesses such as influenza)
• Sepsis (a severe, body-wide response to infection)
• Autoimmune and inflammatory diseases (e.g., rheumatoid arthritis, lupus)
• Inflammatory bowel disease (Crohn’s disease, ulcerative colitis)
• Allergic reactions and asthma flare-ups
• Bone marrow disorders (e.g., aplastic anemia, myelodysplastic syndromes)
• Leukemia and other blood cancers (e.g., chronic lymphocytic leukemia)
• Neutropenia (low neutrophils), including medication-related or chemotherapy-induced neutropenia
• Nutritional deficiencies affecting blood cell production (e.g., vitamin B12 or folate deficiency)

• General wellness and routine health screening as part of a CBC
• Early detection and monitoring of infection when symptoms are unclear
• Tracking immune system status during or after chemotherapy, radiation, or immunosuppressive therapy
• Monitoring chronic inflammatory conditions and autoimmune disease activity
• Evaluating unexplained fatigue, fever, or recurrent illnesses to support timely diagnosis
• Assessing recovery after surgery, injury, or serious illness (watching for complications such as infection)
• Medication safety monitoring for drugs that can lower or raise white blood cell counts