Final answer:
An MHC molecule can present only one peptide at a time; MHC I molecules are found on all nucleated cells, while MHC II molecules are expressed only by professional antigen-presenting cells.
Step-by-step explanation:
An MHC molecule can present only one peptide at a time. Both MHC I and MHC II molecules are involved in the immune response, with each playing a distinct role in antigen presentation. The antigen-binding cleft of the MHC I molecule, formed by the α1 and α2 domains, or the MHC II molecule, formed by the α1 and β2 domains, can bind to one peptide and present it on the surface of cells.
For MHC I molecules, all nucleated cells present peptides derived from intracellular proteins, which are typically self-antigens or, in the case of an infection, pathogenic antigens. MHC II molecules, in contrast, are restricted to professional antigen-presenting cells like macrophages, B cells, and dendritic cells, and they present peptides from extracellular sources.
The specificity of the antigen-binding cleft allows each MHC molecule to present a specific peptide epitope, thus displaying only one at a given time to T cells for the immune response initiation or regulation. Therefore, MHC polymorphism and matching are critical in the context of organ transplantation, with matches potentially reducing the risk of rejection.