White Blood Cells

White blood cells (WBCs), or leukocytes, are the cells of the immune system responsible for defending the body against infections, foreign invaders, and abnormal cells including cancer cells. Unlike red blood cells, which carry oxygen, white blood cells patrol the bloodstream and tissues seeking out and destroying threats. They make up only about 1% of blood volume but are essential for survival — without functional WBCs, even minor infections would be fatal.

There are five main types of white blood cells, each with specialized functions. Neutrophils (the most abundant, 50-70%) are first responders that engulf and destroy bacteria. Lymphocytes (20-40%) include T cells, B cells, and NK cells that drive the adaptive immune response. Monocytes (2-8%) differentiate into macrophages and dendritic cells that engulf pathogens and present antigens. Eosinophils (1-4%) fight parasites and are involved in allergic reactions. Basophils (<1%) release histamine and other mediators of inflammation.

A complete blood count with differential (CBC with diff) measures both total WBC count and the proportions of each type. Normal total WBC count ranges from 4,500 to 11,000 cells per microliter. Elevated WBCs (leukocytosis) often indicate infection, inflammation, or stress, while low WBCs (leukopenia) may indicate bone marrow problems, autoimmune disorders, or immunosuppression.

White blood cell counts are among the most frequently ordered laboratory tests in clinical medicine. Your doctor may order a CBC with differential when evaluating suspected infections, monitoring immune function, investigating unexplained symptoms, or screening for blood disorders. Understanding your WBC results helps you participate in conversations about your immune health.

Persistently abnormal WBC counts — whether elevated or low — warrant further investigation. Elevated counts may indicate infection, inflammatory conditions, medication effects (steroids), or rarely, blood cancers. Low counts may indicate bone marrow suppression, autoimmune destruction, or viral infections. Context including your symptoms, medications, and other lab results is essential for interpretation.