Why do you need your immune system to be working for antibiotics to work? why can't the body use antibiotics properly if the immunity is compromised?

Antibiotics do work: Patients who have their immune systems suppressed by chemotherapy still can be treated with antibiotics. However, your body does a better job of clearing an infection using antibiotics if your immune system is working also properly. If someone with a low immune system gets a very serious infection, antibiotics sometimes aren't enough to save them.

Antibiotics: Very few antibiotics actually kill all organisms present. They often inhibit or reduce the numbers so that your immune system can handle the remainder. If your immune system is compromised the bacteria will grow back out.