Thymic nurse cells (TNCs) are large epithelial cells found in the cortex of the thymus and also in cortico-medullary junction. They have their own nucleus and are known to internalize thymocytes through extensions of plasma membrane. The cell surfaces of TNCs and their cytoplasmic vacuoles express MHC Class I and MHC Class II antigens. The interaction of these antigens with the developing thymocytes determines whether the thymocytes undergo positive or negative selection. Structure and function Thymic nurse cells (TNCs) are a sub-population of cortical thymic epithelial cells (cTECs). pH91, which is a TNC-specific monoclonal antibody, that can be used to identify TNCs. Thymic nurse cells express both MHC Class I and II antigens, and are found in the cortico-medullary junction in addition to the cortex of the thymus. The thymic nurse cells in the cortico-medullary junction express cytokeratin 5 (K5) and cytokeratin 8 (K8), whereas the ones in the cortex express only cytokeratin 8. Thymic nurse cells expressing only cytokeratin 5 have not been identified so far. Hendrix et al. found in their study that one-fourth of the nurse cells isolated from mice were double-positives for K5 and K8, while the rest of them were positive only for K8. The extensions of plasma membrane from thymic nurse cells form a cage-like structure, which trap (Hendrix et al., 2010) triple positive T cells, αβTCRlowCD4+CD8+ within the spaces formed by the interlocking of the membrane. Some of these T cells retain their mobility and undergo maturation to the developmental stage of αβTCRhighCD69+; they are then released from the TNC complex. The enclosed thymocytes have been found to remain intact and retain both metabolic and mitotic activities despite lacking any contact with the extracellular environment. Although initially thought to be involved only in positive selection, thymic nurse cells have now been discovered to facilitate negative selection of thymocytes as well. Negative selection refers to the degradation of thymocytes, and has been found to occur through the help of lysosomes. Lysosomes are present near the nucleus in the cytoplasm of TNCs. If the internalized thymocytes are selected for negative selection, vacuoles containing the thymocytes move closer to the area with lysosomes and eventually fuse with the lysosomes. This leads to the degradation of the T cells within the vacuoles. Macrophages have also been found actively moving in and out of the vacuoles inside the TNCs during the times of high apoptotic activity suggesting their