Figure 2.

T cells develop in the thymus from T-cell precursors, derived from the bone marrow, that enter the cortex of the thymus, where they expand and undergo the process of positive selection, induced by their interaction with thymic epithelial cells of host type A. As they then mature, they move toward the medulla, passing through the corticomedullary junction (C–M junction), where the process of negative selection occurs, largely through interaction with dendritic cells, which enter the thymus from the periphery. Since peripheral dendritic cells of mixed chimeras are of both host and donor types, negative selection occurs for cells reactive with both A and B.