Primer: mechanisms of immunologic tolerance.

Successful adaptive immunity against a broad range of pathogens depends on the diversity of randomly generated T-lymphocyte and B-lymphocyte receptors. A subset of these receptors will be self-reactive and must be regulated to prevent autoimmunity. The process of immunologic tolerance addresses this problem by either purging autoreactive receptors from the system or tuning down their reactivity sufficiently to prevent disease. Immature lymphocytes generate a novel receptor during development in the thymus or bone marrow. Engagement of self antigens by these nascent receptors leads to their purging, either by the apoptotic death of the lymphocyte or by the initiation of receptor editing, a process in which the autoreactive receptor is replaced. If the lymphocytes mature further, the activation threshold of autoreactive cells can be tuned by the co-expression of inhibitory receptors or negative signaling molecules, allowing the persistence of the receptor without an increased risk of autoimmunity. T-cell and B-cell receptors that escape these checkpoints can still be regulated in the peripheral immune system by both purging and tuning mechanisms. A separate set of mechanisms, mediated by various regulatory cells, also operates to tune peripheral receptors in a cell-extrinsic fashion. The combined action of these processes ensures that the organism does not suffer autoimmune pathology, even if autoreactive receptors are generated and maintained in the immune system.