CD4/CD8 ratio

The CD4/CD8 ratio is the proportion of CD4+ helper T cells to CD8+ cytotoxic T cells in peripheral blood, with a healthy reference range typically cited as approximately 1.5–2.5. In young, immunocompetent adults, CD4+ cells predominate and coordinate adaptive responses, whereas CD8+ cells patrol for infected or transformed cells. With age — particularly in CMV-seropositive individuals — oligoclonal CD8+ expansions accumulate, compressing the ratio and sometimes inverting it below 1.0, a pattern associated with frailty, poor vaccine responses, and increased all-cause mortality in elderly cohorts and in studies of HIV-positive individuals. An inverted CD4/CD8 ratio is increasingly used as a component of immune-risk profiling in the context of immunosenescence.