Cellular immunity

class	genetic loci	expression	domains	TCR on T cell	binding cleft
MHC Class I	HLA-A HLA-B HLA-C	all nucleated cells	α₁, α₂ ――― β₂MG, α₃	TCR+CD8 on target cells CD8 binds to α₃	peptide with 8-9 AAs
MHC Class II	HLA-DP HLA-DQ HLA-DR	dendritic cell macrophage B cell activated T cell some epitherial cells in thymus and intestine	α₁, β₁ ――― α₂, β₂	TCR+CD4 on antigen presenting cell CD4 binds to β₂	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

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

*Tfh = follicular B helper T cell