kinda, but the system is not perfect.

https://en.wikipedia.org/wiki/Autoimmune_regulator

oversimplifying but in every person there are tradeoffs about attacking things inside yourself (viruses, cancer, etc) and outside things (parasites, bacteria, etc) and your genetics influences this. See the old Th1/Th2 balance

anyway it was probably a typo on his part, he's such a sleepy guy..