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White blood cells, or leukocytes, are a part of the immune system. They help our bodies fight infection. White blood cells circulate in the blood so they can be transported to an area that has developed an infection. In normal adult bodies there are 4,000 to 10,000 (average 8,000) white blood cells per microliter of blood. White blood cells are found in blood and lymph, cerebrospinal fluid, lymphatic ganglions, connective tissue, and inflammatory effusions, as they are mobile and migratory. Their average dimension varies between 9 and 18 µm. Normal blood values are about 8000 leukocytes per cubic millimeter. 

So, the correct answer is '8000'.