T Cell Development and Migration - T cells are important white blood cells of the immune system. - T cells play a central role in the adaptive immune response. - T cells have a T-cell receptor (TCR) on their cell surface. - T cells are derived from hematopoietic stem cells in the bone marrow. - Developing T cells migrate to the thymus gland to mature. - After migration to the thymus, precursor cells differentiate into distinct types of T cells. - T cell differentiation continues even after leaving the thymus.

T Cell Subtypes - CD8+ T cells are cytotoxic and can directly kill virus-infected cells and cancer cells. - CD4+ T cells function as helper cells. - CD4+ T cells activate memory B cells and cytotoxic T cells. - CD4+ T cells secrete different types of cytokines depending on their subtype. - Examples of different T cell subtypes include T-helper1, T-helper2, T-helper17, and regulatory T cells.

Regulatory T Cells - Regulatory T cells provide the critical mechanism of tolerance. - Regulatory T cells prevent immune cells from reacting against one's own cells. - Regulatory T cells are also known as suppressor T cells. - Regulatory T cells can be co-opted by cancer cells to prevent immune response against tumors. - Regulatory T cells play a role in preventing autoimmune responses.

TCR Development and Positive Selection - TCR development is a critical step in T cell maturation. - Each mature T cell contains a unique TCR that reacts to different pathogens. - TCR consists of alpha and beta chains. - Thymocytes undergo positive selection in the thymic cortex. - Positive selection ensures that T cells can recognize self-MHC molecules. - Thymocytes that weakly bind to self-MHC molecules survive positive selection.

Negative Selection and Self-Tolerance - Negative selection occurs in the thymic medulla. - Thymocytes that strongly bind to self-MHC molecules are removed. - Self-antigens are presented on the MHC complex of medullary thymic epithelial cells (mTECs). - Thymocytes that interact too strongly with self-antigens receive an apoptotic signal. - Thymic dendritic cells play a role in presenting self-antigens to developing T cells. - The process of negative selection ensures the development of self-tolerant T cells.