Could you tell me what are good foods to eat while you're on antibiotics to boost your immune system back up?

Probiotics. Antibiotics don't just kill the bacteria that make you sick, they kill bacteria that protect your guts, your mouth, your genitals etc. By killing good bacteria, antibiotics create the potential for bad germs of various kinds to grow. By taking probiotics, you're replacing the good bacteria being killed with lots more good bacteria. Yogurt, kefir, and supplements can all help.
Probiotics help. Many people are looking for some "magic pill" to "boost their immune system". There is no such pill. However, there is a way to boost your immune system, live a healthy lifestyle. Proper diet, proper sleep- wake cycles, healthy environment, proper exercise are the foundation of good health and good immune health. Probiotics should be used after antibiotics to replace "good" bacteria in your gut.