Final answer:
Helper T cells lack the CD8 coreceptor needed to bind to MHC I molecules, hence they cannot respond to antigens presented by MHC I.
Step-by-step explanation:
Helper T cells cannot respond to antigens (Ags) presented by MHC I molecules because they lack the coreceptor CD8. Helper T cells, or CD4+ T cells, require antigens to be presented by MHC class II molecules for proper activation. These cells are activated when they engage with an antigen presented on MHC II molecules that are found only on the surface of antigen-presenting cells, including macrophages, dendritic cells, and B cells. In contrast, cytotoxic T lymphocytes (CTLs), or CD8+ T cells, are designed to interact with MHC I molecules, which can be presented by all nucleated cells.