This chapter begins with a description of the role of the white blood cell in killing foreign invaders. White blood cells are responsible for destroying foreign organisms, such as viruses and bacteria. They are also responsible for killing non-white blood cells, including those that have a genetic defect that causes them to multiply uncontrollably. This chapter ends with a discussion of the roles of white blood cells and the lactic acid bacterium, which is one of the most important organisms in the human body.