The immune system has evolved the capacity to react specifically with a very large number of foreign molecules with which it had no previous contact while avoiding reactivity for autologous molecules, naturally antigenic in other species or in other individuals of the same species.
As biologists, we contemplate with admiration and awe the wondrous array of sophisticated cell interactions and recognitions evolved in the T cell immune system, which must be a model for other similarly complex biological systems of highly differentiated organisms.