This review covers T-cell subgroups with a focus on pathogenic Th1 and Th17 cells and their role in mediating neuronal death and demyelination in multiple sclerosis (MS) and stroke, as well as the neurotoxic nature of CD8+ cytotoxic T lymphocytes and the neuroprotective and immunosuppressive roles of regulatory T cells (Tregs) that promote CNS homeostasis. The gene discussed is CD8A; the disease is multiple sclerosis.