Where we used to go to church Linda taught Sunday school with a pediatrician. I was asking him once about childhood illnesses and the various school choices. He said that there are really only about 20 viruses that kids generally get throughout their childhood. The kids in daycare at an early age usually get them all before the age of two. Public and Private schooled kids get them later, but fairly quickly.  Homeschooled kids get them more slowly, but almost always all of them by the time they’re in their teens. He said the real danger is for the daycare kids because their immune system isn’t developed yet and getting sick that often isn’t good for them at that age.

So homeschooling doesn’t leave kids with "unprepared" immune system, in fact just the opposite, it allows them to handle the illnesses at a slower pace, and develop the immunity without that bad side affects that would occur if they were younger.