The role of the immune system may be succinctly stated as the "preservation of integrity". This system is responsible for identifying what is "self" and what is "non-self". The great complexity of the mammalian system is an indication of the importance, as well as the difficulty, of this task. If the system fails to recognize as non-self an infectious entity or the neoantigens expressed by a newly arisen tumour, then the host is in danger of rapidly succumbing to the unopposed invasion.