Question

Getting enough rest and eating healthy foods help to maintain a healthy immune system.

Answer 1

Getting enough rest and eating healthy foods are indeed two of the best strategies how one might want to maintain a healthy immune system. When we would add the addition of working out from time to time, we would have a person with a very healthy immune system. 

Answer 2

That statement is true

The strength of our immune system is determined by our white blood cells.

Getting enough rest will make our blood cells regenerate faster ,  and eating healthy foods ( especially those that are rich in vitamin C) will increase the productivity of our white blood cells