How does chemotherapy affect your immune system?
During chemotherapy, your immune system will weaken so that the treatment can attack your rapidly growing cancer cells. But it affects other places with fast-spreading cells as well, like your hair, mouth, etc. Since your immune system is weakened, you can't keep those good cells growing like they normally do.
How does chemotherapy affect your white blood cells?
Chemotherapy has already weakened your immune system, so now your left defenseless against the merciless cancer treatment. Since your white blood cells are cells that grow fast, the chemotherapy will destroy those. Destroying your white blood cells will lower your blood count, and if your blood count is lower than is normally should, doctors will stop treatment and try to prevent diseases that may come to you in your defenseless state.