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.