Cellular immunity
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MHC = HLA
- Major Histocompatibility Complex (MHC) is rephrased as Human Leukocyte Antigen (HLA)
- Genetic loci are on 6th chromosome
MHC and T cell receptor (TCR)
- MHC presents peptide antigen and TCR recognize it
- CD8 acts as co-receptor to MHC Class I on all target cells
- CD4 acts as co-receptor to MHC Class II on antigen-presenting cells
class | genetic loci | expression | domains | TCR on T cell | binding cleft |
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MHC Class I |
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α1, α2 ――― |
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peptide with 8-9 AAs |
MHC Class II |
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α1, β1 ――― |
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peptide with 18-20 AAs |
MHC Class III |
T cell development
- Prothymocytes migrate from bone marrow to thymus
- Prothymocytes proliferate and become Thymocytes in thymus
- Thymocytes express all of CD3, CD4 and CD8 (Double positive thymocytes)
- DP thymocytes are challenged to bind peptide-MHC (Positive selection)
- CD8+ thymocytes are to suppress CD4 expression, vise versa (Single positive CD8 or CD4 cells)
- SP cells encounter Epitherial reticular cells (APC - DC, Macrophage) and challenged to bind too efficiently to pMHC I or pMHC II (Negative selection)
- SP cells which do not bind too efficiently to pMHC I or pMHC II exit thymus as (naive) T cells
T cell activation by 2 signaling (2 signal model)
- Naive T cell needs doubled signals of TCR-pMHC interaction and co-stimulatory molecules by APC to be activated
- Co-stimulatory molecules: CD28, ICOS, CTLA-4 and PD-1
- CD28 and ICOS on T cell acts as activation signal
- CTLA-4 and PD-1 on T cell acts as inhibitory signal against activation
CD8+ T cell functions
- Cytotoxicity
- Killing virus-infected cells, malignant cells
- Perforin/granzyme-mediated
- Fas/Fas ligand-mediated
- IFN-γ production
CD4+ T cell differentiation
- *Tfh = follicular B helper T cell