In their Nobel Lecture, Medawar and Burnet define immune tolerance as "a state of indifference or non-reactivity towards a substance that would normally be expected to excite an immunological response."[REF]
Immune tolerance is formally differentiated into central or peripheral;[REF] however, alternative terms such as "natural" or "acquired" tolerance have at times been used to refer to establishment of tolerance by physiological means or by artificial, experimental, or pharmacological means [REF].