Why do the doctor check your white and red blood cells?

Many reasons: Most common reason to look at white cells is concern for a bacterial infection. Most common reason to look at red cells involves anemia. Why don't you ask your doctor? There are many other reasons one might check them.

White and red cells: Doctors check your white cells to see if you have any signs of infection or inflammation in your body. Red cells are what carry oxygen to all your organs. If you don't have enough you have anemia and will feel very tired.

Different things.: Red cells tell us, among other things, if you are getting enough iron (or if you are losing blood somewhere.) white cells tell us (roughly) what kind of infection you may be having if you have a fever.