How does the immune system know where "our own" and where "alien" are? Not the easiest task🤔 So it is precisely because of this complexity that this issue is regulated at the gene level. Those cells that the immune system comes in contact with in a healthy body are put on their "friend list" - they remain untouched. Again, this info is sewn into the genes. But foreign particles and microorganisms are not on this "good" list. So, immune cells notice this and send out a "rapid response team". The immune system gives the command to absorb the pathogen or destroy it completely.