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White Blood Cell (WBC)

White Blood Cells (WBCs), or leukocytes (also spelled "leucocytes"), are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Leukocytes are found throughout the body, including the blood and lymphatic system. The number of WBCs in the blood is often an indicator of disease. There are normally between 4×109 and 1.1×1010 white blood cells in a liter of blood, making up approximately 1% of blood in a healthy adult. An increase in the number of leukocytes over the upper limits is called leukocytosis, and a decrease below the lower limit is called leukopenia.