White blood cell countWBC · Leukocytes

The white blood cell count (WBC) is the total number of immune cells circulating in a volume of blood. These cells are the body's front line against infection, and their number rises and falls with what the immune system is dealing with.

It matters because both unusually high and unusually low counts are informative — a high count often signals infection or inflammation, a low count can reflect certain illnesses or treatments — and the baseline level is one of the markers used in biological-age scoring.

In practice, WBC is read as part of the full blood count, and the differential (the mix of white-cell types, including lymphocytes) adds far more meaning than the total alone.