Final answer:
The co-stimulatory molecule distinguishing P-APCs from other cells is B7. CTLs bind MHC I molecules via CD8. MHC I is found on all nucleated cells, while MHC II is limited to P-APCs.
Step-by-step explanation:
The co-stimulatory receptor/molecule that distinguishes professional antigen-presenting cells (P-APCs) from other cells is B7. The expression of B7 on APCs like dendritic cells, macrophages, and B cells provides a necessary second signal to T cells, which along with the primary signal from the MHC-antigen complex, leads to the activation of T cells.
Regarding the immune cell interaction with MHC molecules, cytotoxic T lymphocytes (CTLs) are the immune cells that bind MHC I molecules on APCs via CD8 coreceptors on their cell surfaces.
All nucleated cells in the body express MHC I molecules, which present antigen peptides to CD8+ cytotoxic T cells. In contrast, MHC II molecules are found only on macrophages, dendritic cells, and B cells and are responsible for presenting antigens to CD4+ helper T cells.