Sources differ slightly on what white blood cell count (WBC) range is considered normal for adults. The National Institutes of Health define a normal adult range as 4,500-10,000 white blood cells per microliter, and the Mayo Clinic defines a normal adult range as 3,500-10,500 white blood cells per microliter.
Wikipedia cites sources that variously define lower limits for adults from 3,500 to 4,500 cells per microliter and upper limits for adults from 9,000 to 11,000 cells per microliter. The National Institutes of Health note that some differences in normal ranges may be attributable to varying laboratory practices.
Healthline notes that normal ranges for infants are higher than the normal ranges for adults, with Wikipedia giving the upper limit for newborns as 30,000 cells per microliter.
White blood cells are immunity cells. They help fight off infections that arise from foreign pathogenic bacteria or viruses. Examples of white blood cells include basophils, eosinophils, lymphocytes, monocytes and neutrophils. A total white blood cell count measures the total amount of these five types of white blood cells. However, each of the five can be measured separately. A high white blood cell count is called leukocytosis, and causes include cigarette smoking and intense stress.