What are White Blood Cells?

White blood cells (WBC), also called leukocytes, are cells of the immune system that defend your body against infections, viruses, bacteria, and other foreign invaders. There are five main types: neutrophils, lymphocytes, monocytes, eosinophils, and basophils — each with a specialized role in immune defense.

Why is it Important?

The WBC count is a fundamental measure of immune system health. Abnormal levels can indicate infections, immune disorders, bone marrow problems, or allergic reactions. A WBC count is part of the complete blood count (CBC), one of the most frequently ordered blood tests.

Normal Ranges

Group	Range	Unit
Adult	4,500 – 11,000	cells/µL
Adult	4.5 – 11.0	×10⁹/L
Children (2–12 yr)	5,000 – 13,000	cells/µL
Newborn	9,000 – 30,000	cells/µL

What Do Abnormal Results Mean?

High WBC (Leukocytosis)

Common causes include:

Bacterial or viral infections
Inflammatory conditions (rheumatoid arthritis, IBD)
Allergic reactions
Stress (physical or emotional)
Smoking
Certain medications (corticosteroids, epinephrine)
Leukemia and other blood cancers

Low WBC (Leukopenia)

Common causes include:

Viral infections (influenza, HIV, hepatitis)
Bone marrow disorders (aplastic anemia, myelodysplasia)
Autoimmune diseases (lupus, rheumatoid arthritis)
Chemotherapy or radiation therapy
Severe infections (sepsis — consumes WBCs)
Certain medications (antibiotics, anticonvulsants)
Nutritional deficiencies (B12, folate, copper)

When Should You Get Tested?

WBC count is part of a standard CBC panel, often included in routine health checkups. Your doctor may specifically order it if you have signs of infection (fever, chills), unexplained fatigue, frequent infections, bruising, or if you are undergoing treatments that affect bone marrow.