Immunologists called the self-reactive cells “forbidden clones”—forbidden because they had dared to react to some aspect of a self peptide and were therefore deleted from existence before they could be allowed to mature and attack the self. Burnet likened them to “holes” in immune reactivity. It is one of the philosophical enigmas of immunity that the self exists largely in the negative—as holes in the recognition of the foreign. The self is defined, in part, by what is forbidden to attack it. Biologically speaking, the self is demarcated not by what is asserted but by what is invisible: it is
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