I was wondering is it possible to be immune to cavities?

No immunity: No. Interestingly enough, some patients tell me that they brush 10 times a day, eat nutritious food ; still get cavities all the time. Others don't take good care of their teeth, seldom come for dental check-ups ; still have no or minimal cavities or gum problems. You can't be "immune" to getting cavities but some have better genes ; are just lucky. 99% have to brush\floss and watch what they eat.

Not immune,resistant: Cavities are caused by the combination of normal oral bacteria, the sugars we ingest (cards become sugar as we digest them) and failure to effectively keep the oral cavity clean. Many factors play a roll such as nutrients, oral hygiene, fluoride , professional visits, oral saliva acidity, bacteria population and even genetics. So you can be cavity prone, resistant or in the middle.