White blood cells are cells adapted to combat against infectious disease and any other foreign material that may enter the body. They are adapted to change shape, allowing them to wrap around microbes in the body and engulf them. Once microbes are engulfed they can be digested by the white blood cell and this helps the immune system defend the body from further attack.
The ability of white blood cells to change shape also allows them to squeeze through capillary walls to reach microbes in tissues and organs.
White blood cells come in different forms, the two most common being lymphocytes and phagocytes.