A key difference between MHC-I and MHC-II molecules is:

Question

BlackTom AI · Accepted Answer

The question asks us to identify a key difference between MHC-I and MHC-II molecules.
Option 1: 'Only MHC-I genes are polymorphic' is incorrect. In reality, both MHC-I and MHC-II genes are highly polymorphic, contributing to broad antigen presentation diversity across individuals.
Option 2: 'The a2 chain participates in peptide-binding on MHC-II molecules only' is misleading. While the MHC-II peptide-binding groove involves regions from both the alpha and beta chains (particularly the alpha1 and beta1 domains), saying only the a2 chain participates oversimplifies the binding site and misstates which chain is involved.
Option 3: 'The Beta chain participates in peptide-binding on MHC-II molecules only' is correct. In MHC-II, the beta chain (beta1 domain) contributes to the peptide-binding groove, whereas in MHC-I the beta2-microglobulin does not participate in peptide binding. This captures a fundamental distinction in where binding interactions occur for MHC-II versus MHC-I.
Option 4: 'MHC-II is expressed on all cells, whereas MHC-I expression is restricted' is false. MHC-I is expressed on nearly all nucleated cells, whereas MHC-II expression is limited to professional antigen-presenting cells (like dendritic cells, macrophages, B cells) and some other specialized cell types under certain conditions.

Putting this together, the statement that correctly highlights a key difference is that the beta chain participates in peptide-binding on MHC-II molecules, which sets MHC-II apart from MHC-I in terms of peptide-binding involvement.

A key difference between MHC-I and MHC-II molecules is:Single choice

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