White blood cells do not attack self body cells because they are able to distinguish between self and non-self cells. This is a role played by the aquired immune system. To accomplish this, resident cells or self cells express proteins that mark them as part of the host, distinguishing them from invaders. Expressing these proteind isn't enough to simply identify self cells; in addition, effective cooperation between cells must occur so that efficient information sharing and selective effector activities occur. In humans, this system is called the major histocompatibility complex.