Final answer:
T cells require more specific peptide:MHC complexes for activation in the absence of a co-receptor (CD4 or CD8). These co-receptors stabilize the interaction between T cell receptors and MHC molecules, enhancing the immune response.
Step-by-step explanation:
In the absence of a co-receptor (CD4 or CD8 for T helper cells or cytotoxic T cells, respectively), T cells require more specific peptide:MHC complexes on the antigen-presenting cell compared with interactions involving a co-receptor. This is because the co-receptor helps to stabilize the interaction between the T cell receptor, the MHC molecule, and the peptide. Without the assistance of the co-receptor, a greater number of peptide:MHC complexes is needed to ensure sufficient signaling for T cell activation.
Co-receptors CD4 and CD8 facilitate the binding of T cells to MHC II and MHC I molecules, respectively. CD4+ T cells, or helper T cells, interact with MHC II molecules typically found on professional antigen-presenting cells. Conversely, CD8+ T cells, or cytotoxic T cells, engage with MHC I molecules that are present on all nucleated cells.
It's important to note that the information suggesting MHC II is found on most body cells and MHC I on immune cells only is incorrect. Rather, MHC I is found on all nucleated cells and is presented to CD8+ cytotoxic T cells, while MHC II is generally restricted to professional antigen-presenting cells and is recognized by CD4+ helper T cells.