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Foreign antigens are presented in secondary lymphoid sites
• Think back to T cell development:
• In this generative tissue, usually only self antigens are present
Rearrangement of T cell receptor genes
Express low level of TCR Immature progenitors are resistant
to apoptosis Low levels of FAS
High levels of Bcl-2 (anti-apoptotic) Double positive - CD4 & CD8
Double-positive High levels of FAS
Very low Bcl-2 Highly sensitive to
apoptosis!!!!
Thymic Selection
T lymphocyte ***
• Positive Selection
• Negative Selection
Cortical Epithelial Cells
Recognition of self MHC
Also called MHC restriction
MCH + antigen!!
T cell receptor diversity
All T cells will have receptors that recognize MHC + antigen
Thymic dendritic cells general self antigens
Short lived
Medullary epithelial cells tissue specific antigens
Thymic Selection
HOW CAN THIS WORK?
1. TCRs must recognize MHC+self antigen to pass through positive selection. 2. TCRs must mediate negative selection - ie. Self tolerance. 3. Later, must also signal activation when faced with foreign peptides.