CD4+/CD8+ ratio
The CD4/CD8 ratio is the ratio of T helper cells (with the surface marker CD4) to cytotoxic T cells (with the surface marker CD8). Both CD4 and CD8 T cells contain several subsets. The CD4/CD8 ratio in the peripheral blood of healthy adults and mice is about 2:1, and an altered ratio can indicate diseases relating to immunodeficiency or autoimmunity. An inverted CD4/CD8 ratio (namely, less than 1/1) indicates an impaired immune system. Conversely, an increased CD4/CD8 ratio corresponds to increased immune function. Obesity and dysregulated lipid metabolism in the liver leads to  loss of CD4, but not CD8 cells, contributing to the induction of liver cancer. Regulatory CD4 cells decline with expanding visceral fat, whereas CD8 T-cells increase. Decreased ratio with infection. A reduced CD4/CD8 ratio is associated with reduced resistance to infection. Patients with tuberculosis show a reduced CD4/CD8 ratio. HIV infection leads to low levels of CD4 T cells (lowering the CD4/CD8 ratio) through a number of mechanisms, including killing of infected CD4. Acquired immunodeficiency syndrome (AIDS) is (by one definition) a CD4 T cell count below 200 cells per µL. HIV progresses with declining numbers  of CD4 and expanding number of CD8+ cells (especially CD8 memory cells), resulting in high morbidity and mortality. When CD4 T cell numbers decline below a critical level, cell-mediated immunity is lost, and the body becomes progressively more susceptible to opportunistic infections. Declining CD4/CD8 ratio has been found to be a prognostic marker of HIV disease progression. COVID-19. In COVID-19 B cell, natural killer cell, and total lymphocyte counts decline, but both CD4 and CD8 cells decline to a far greater extent. Low CD4 predicted greater likelihood of intensive care unit admission, and CD4 cell count was  the only parameter that predicted length of time for viral RNA clearance. Decreased ratio with aging. A declining CD4/CD8 ratio is associated with ageing, and is an indicator of immunosenescence. Compared to CD4 T-cells, CD8 T-cells show a greater increase in adipose tissue in obesity and aging, thereby reducing the CD4/CD8 ratio. Amplication of numbers of CD8 cells are required for adipose tissue inflammation and macrophage infiltration, whereas numbers of CD4 cells are reduced under those conditions. Antibodies against CD8 T-cells reduces inflammation associated with diet-induced obesity, indicating that CD8 T-cells are an  important cause of the inflammation. CD8 cell recruitment of macrophages into adipose tissue can initiate a vicious cycle of further recruitment of both cell types. Elderly persons commonly have a CD4/CD8 ratio less than one. A study of Swedish elderly found that a CD4+/CD8+ ratio less than one was associated with short-term likelihood of death. Immunological aging is characterized by low proportions of naive CD8 cells and high numbers of memory CD8 cells, particularly when cytomegalovirus is present. Exercise can reduce or reverse this effect, when not done at extreme intensity and duration. Both effector helper T  cells (T1 and T2) and regulatory T cells (T) cells have a CD4 surface marker, such that although total CD4 T cells decrease with age, the relative percent of CD4 T cells increases. The increase in T with age results in suppressed immune response to infection, vaccination, and cancer, without suppressing the chronic inflammation associated with aging.