Positive selection of thymocytes: the long and winding road.

Positive selection is a crucial stage in T-cell development because it is here that CD4+CD8+ cells bearing T-cell receptors that interact with self-major histocompatibility complex molecules are rescued from cell death, resulting in the generation of mature T cells. Here, Graham Anderson and colleagues review recent studies indicating that positive selection is a multistage process involving interactions with thymic epithelium.