Why White Blood Cell Counts Go Up—or Down

White blood cells (WBCs) are an essential part of your immune system, acting as your body’s rapid response team against infections, inflammation, stress, and other threats. A high or low white blood cell count is a common finding on blood tests, but it can mean many different things depending on your symptoms and overall health.

What Is a Normal WBC Count?

Most adults have a WBC between 4,000 and 11,000 cells per microliter. But the “normal” range can vary depending on the lab and on factors such as age, medications, and medical history.

Causes of a High WBC Count (Leukocytosis)

Infections: The most common cause. Bacterial infections typically cause higher increases than viral infections.

Inflammation: Conditions like rheumatoid arthritis or chronic autoimmune disease can raise WBC levels.

Stress or Steroids: Physical stress (exercise, surgery, trauma) and steroid medications can temporarily increase WBCs.

Smoking:Often causes a mild but persistent elevation.

Bone Marrow Conditions: Rarely, leukemia or other bone marrow disorders cause high WBC counts.

Causes of a Low WBC Count (Leukopenia)

Viral infections: Flu, COVID-19, and other viruses may suppress WBC production temporarily.

Autoimmune diseases: Some conditions can reduce the number of white blood cells.

Medications: Certain antibiotics, seizure medications, and chemotherapy can lower WBC counts.

Bone marrow disorders: Aplastic anemia, myelodysplastic syndromes, and leukemia may cause low counts.

Do Symptoms Matter?

Yes. The meaning of an abnormal WBC count depends heavily on your symptoms:

• Fever, chills, or sweats

• Fatigue

• Frequent infections

• Easy bruising

• Unexplained weight loss

Symptomatic abnormalities are more concerning than isolated lab findings.