Final answer:
MHC class I molecules typically bind nonamer peptides and are present on all nucleated cells, while MHC class II molecules are expressed on professional antigen-presenting cells and interact with CD4+ helper T cells.
Step-by-step explanation:
The question pertains to the binding characteristics of Major Histocompatibility Complex (MHC) class I molecules. MHC class I molecules are known for binding mostly nonamer peptides (peptides made up of nine amino acids), which fit within the antigen-binding cleft formed by the α1 and α2 domains. The peptides presented by MHC class I molecules are typically derived from the cytoplasm of cells and are presented to CD8+ T cells, which are part of the cellular immune response.
On the contrary, MHC class II molecules are found only on professional antigen-presenting cells such as macrophages, dendritic cells, and B cells. These molecules bind to CD4 molecules on T cells and are involved in initiating the immune response by presenting processed extracellular proteins to helper T cells.
To summarize, MHC class I molecules bind nonamer peptides and are present on all nucleated cells, while MHC class II molecules are restricted to professional antigen-presenting cells and engage with helper T cells.