Can having good immune system help prevent cavity and gum disease?

Cavities, no: Cavities are caused by the acids excreted by the bacteria that reside in your mouth and stick to your teeth. This is not an infection. Periodontal disease is caused by an irritation of the support systems of your teeth (bone and gums) again, by bacteria stuck to the root surface and in the sulcus. A good immune system can help ward off infection, but proper brushing and flossing is best bet ..

Absolutely, but: Dental disease is a mostly behavioral issue: a result of abuse and neglect. The abuse is high carb/sugar food and drink, dietary and gastric (stomach) acid and toxins from tobacco. The neglect stems from inadequate, inefficient and/or absent excellent oral hygiene. Top it off with the fact that 50% of the population does not see a dentist for preventive care. The immune system can't do it all!

No: Cavity is not an infection but rather an acid demineralization of enamel as food trap becomes ferment and lower the ph of the area for the demineralization to occur. In gum disease, bone & periodontal tissue destruction caused by bacteria/products, your host reaction, and poor occlusion. Good immune system only controlled the extent of infection but the bone loss still occur due to host reaction.