White blood cell count (WBC) measures the total number of immune cells circulating in the blood. The typical reference range is about 4,500–11,000 cells/μL. It serves as a broad marker of immune system activity rather than a diagnosis by itself.

Higher WBC levels can be seen with infection, inflammation, stress, smoking, steroid exposure, or, less commonly, hematologic disease. Lower levels may reflect viral illness, medication effects, autoimmune conditions, or bone marrow suppression. In otherwise healthy people, mild shifts are common and often temporary.