[ChrisV]

The problem with your argument is that natural selection is more or less an all or nothing proposition. You can't have most of an organism created through natural selection and then bits tacked on later by a designer.

Take your example of the immune system. How would that work? Were humans wandering around for a while with no immune system and then suddenly pow! fully functioning immune system? If so, how did they survive with no immune system?

If organisms evolve at all, it seems silly to be to imagine bits suddenly tacked on. That suggests that beforehand, they were a functioning organism apart from not having that specific bit. But this then suggests that the bit is non essential, so it seems to me impossible that crucial pieces of an organism could be added in this way.