Final answer:
MHC I molecules present peptides from intracellular pathogens and have a closed peptide-binding cleft. MHC II molecules present antigens from extracellular pathogens and have an open-ended cleft that allows longer peptides to extend out.
Step-by-step explanation:
The difference between peptides presented via MHC I and MHC II primarily lies in their structure and the type of antigens they present. MHC I molecules are composed of a longer α protein chain coupled with a smaller β₂ microglobulin protein, and they present antigens from infected host cells or intracellular pathogens.
MHC II molecules are made up of two similar-length protein chains, an α, and a β chain, and are responsible for presenting antigens from extracellular pathogens. The peptide-binding cleft of MHC I is closed at both ends, formed by the α₁ and α₂ domains, which restrict the length of the peptides that can bind.
In contrast, the peptide-binding cleft of MHC II molecules, formed by the α₁ and Β₁ domains, is open at both ends, allowing antigen/peptides to "hang out" which enables the binding of longer peptides.