White blood cells are part of the immune system. They usually fight infections and attacks viruses and bacteria. A white blood cell count measures the number of white blood cells in one's body. This helps to check of an inflammatory disorders or other diseases. There are 4x109 and 11x109 white blood cells in 1 liter of healthy adult blood that is about 7000 to 25000 WBC per drop out of the 37.2 trillion cells in the body.