In this chapter, the reader is introduced to the concept of the platelet, a blood cell that is a component of the body's immune system. It is a member of the white blood cell family, which is composed of monocytes, macrophages, and T-cells. The white blood cells are able to fight off Staphylococcus aureus, a common cause of skin infections and food poisoning. The reader is also introduced to a number of other immune cells, including red blood cells, which are also involved in the fight.