The chapter opens with a description of blood cells, which are born in the womb and spent their early childhoods in the blood vessels of the body. A progenitor cell is a type of cell that does not yet differentiate into a specific type of blood cell. A red blood cell, on the other hand, is born as a progenitor cell and is raised by macrophages. The chapter ends with a discussion of the dangers of infection by the bacterium Streptococcus, which can cause blood vessel infections.