The population of MHC-restricted thymocytes that survive positive selection comprises some cells with low-affinity receptors for self-antigen presented by self-MHC molecules and other cells with high-affinity receptors. The latter thy-mocytes undergo negative selection by an interaction with thymic stromal cells. During negative selection, dendritic cells and macrophages bearing class I and class II MHC molecules interact with thymocytes bearing high-affinity receptors for self-antigen plus self-MHC molecules or for self-MHC molecules alone (see Figure 10-5). However, the precise details of the process are not yet known. Cells that experience negative selection are observed to undergo death by apoptosis. Tolerance to self-antigens encountered in the thymus is thereby achieved by eliminating T cells that are reactive to these antigens.
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