Your blood has special cells in it that fight disease and infections. These cells, called white blood cells, are produced in your bone marrow, the, soft tissue that fills the inside of your bones. White blood cells work like an army to attack and kill harmful germs that get into your body.
When an infection develops on your skin, for example, the bone marrow produces more white blood cells than usual. These white cell “soldiers” move to the area where the germs are and actually chew them up and destroy them. They also eat away at the tissue around the germ, and soften and liquefy it. The white cells then break apart and, along with the destroyed germs and liquefied tissue, form the thick yellow-white substance called pus. The pus oozes out of the infection slowly and dries up as the wound heals.
Even though white blood cells are necessary to the body, too many of them can cause a disease called leukemia, a cancer of the blood!