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MHC class I binding cleft is formed by _______
α1 and α2 domains (α chain only)
MHC class II binding cleft is formed by ______
α1 and β2 domains (α and β chains)
CD8 molecules bind ____
α3 domain of the MHC class I molecule
CD4 molecules bind ____
β2 domain of MHC class II molecule
sequence motif
set of amino acids that allows peptide to bind to MHC binding cleft
often 2nd-3rd a.a. (amino terminal end); 9th a.a. (carboxy terminal end)
anchor residue
amino acids that make up sequence motifs
side groups of a.a. insert into pockets in floor of peptide binding cleft and anchors peptides in
features of peptide binding to MHC molecules
each MHC molecule displays one peptide at a time
low affinity, broad specificity → many different peptides can bind to the same MHC molecules
very slow off-rate → displays long enough to be located by T cell
binds peptides only!
do not discriminate between foreign antigens and self antigens
how does the number of class I MHC molecules differ in heterozygous vs. homozygous offspring?
heterozygous = maximum of 6 different class I MHCs → present wider array of peptides
homozygous = 3 class I MHCs → may not be able to respond to certain antigens
what is the pattern of expression for MHC molecules?
co-dominance→ both parental alleles of each MHC gene are expressed
why is high polymorphism of MHC alleles in a population advantageous?
promotes species survival
increases variation in susceptibility to pathogens
in largely homozygous populations, there can be high susceptibility to certain diseases
how are T cells selected for in the thymus?
positive selection for weak recognition of self class I and class II MHC-peptide complexes
failure to recognize MHC-peptide complex → apoptosis
negative selection against strong recognition of either self class of MHC-peptide complex → apoptosis