MHC: important in antibody and cell mediated immunityTCR unique to antigen but recognizes it in combination with MHCMHC is not specific, can bind and display most peptidesImplication of MHC in infectious diseases/autoimmune diseasesMHC as Ag presenting structureDifferent MHC molecules in individuals Variation in Ag presentation to Th and Tc cells Variation in immune responses in individuals

MHC: as transplantation antigensDifferent type of MHC molecules among individualsDetermines compatibility between the tissues of individualsControl immune response through recognition of self and non-selfThus serve as targets in transplantation rejection

MHC is a collection of genes for the expression of surface molecules (proteins) involved in adaptive immune responseMouse: H2 Humans: HLA

Class I MHCExpressed in almost all nucleated cellsDisplay endogenous antigens:display antigens/peptides synthesized with in the altered self cells Display peptides to Tc cells

3 regions: HLA: C, B, and A are involvedConsist of 2 polypeptides: long () and short (2-microglobulin)3 and 2 carry conserved constant sequencesThe variable sequences clustered in 7 subsequences, 3 in 1 domain and 4 in 2 domain.

Class II MHCExpressed on APCDisplay exogenous peptides (from phagocytized proteins) Display to Th cellsConsist of 2 polypeptides ( and )D region: HLA: DP, DQ, DR

Class III MHCEncode several soluble components serum proteins, TNF, etc. which are related to the immune systemEncoded by regions C4, C2, BF

Structure of TCR3 hypervariable regions in both chainsRecognize Ag in association with a self molecule (i.e. MHC)Signal is passed to CD3 and then T cell, prompting T cell activation releasing lymphokines

ANTIGEN RECOGNITION BY T-CELLS TCR - APC (class II) TCR Target cell (class I)