Final answer:
CD8+ T cells require a stronger co-stimulation than Th cells, since their activation typically involves a more complex set of interactions, indicative of their role in directly killing infected cells.
Step-by-step explanation:
CD8+ T cells require a stronger co-stimulation than Th (T helper) cells. This is because the primary role of CD8+ T cells is to kill cells that are infected with viruses or other pathogens. They are often referred to as cytotoxic T cells. CD8+ T cells recognize peptides presented by Major Histocompatibility Complex (MHC) class I molecules, which are found on nearly all nucleated cells.
By contrast, Th cells are helper cells and are CD4+. They typically require antigens to be presented with MHC class II molecules, which are found on professional antigen-presenting cells. Therefore, CD8+ T cells often need a more robust signal to be fully activated, which additionally includes interactions with other molecules and stronger co-stimulatory signals.