What kind of peptides do MHC-II antigens display?

• A. Peptides originating from within the body
• B. Peptides that come from outside the body
• C. Proteins exclusively from viral infections
• D. Only mutated peptides from tumor cells

Answer: (B)

MHC-II antigens are particularly involved in presenting peptides that originate from outside the body. This mechanism is crucial in the immune response, specifically for the activation of CD4+ T cells. MHC-II molecules predominantly interact with extracellular antigens, which can include proteins from pathogens such as bacteria and other foreign substances that are internalized by antigen-presenting cells (APCs) through processes like phagocytosis.

Once these extracellular proteins are engulfed, they are processed within the APCs to produce smaller peptide fragments. These fragments are then loaded onto MHC-II molecules, which transport them to the surface of the cell. When CD4+ T cells recognize these MHC-II-peptide complexes, they become activated and stimulate an appropriate immune response, aiding in the defense against the encountered pathogens.

The correct understanding of MHC-II functions underscores their vital role in adaptive immunity, distinguishing them from other components that present intracellular antigens or mutations associated with tumor cells. This specificity is what makes option B the correct answer in the context of MHC-II antigen functionality.