The results of previous work from our laboratories have suggested that free radical damage to T cells as they age may contribute to the age-related decline in the T cell-mediated immune response. The aims of this investigation were to assess the efficiency of in vivo antioxidant capacity through determining the antioxidant capacity of plasma using the ferric reducing ability of plasma assay, and to assess the levels and types of DNA damage (as a measure of in vivo antioxidant efficiency) using the alkaline comet assay and two enzymatic modifications of the comet assay, in peripheral blood mononuclear cells (PBMCs) from nonagenarian subjects drawn from the Swedish NONA Immune Study. The results obtained were compared with those from middle-aged (40–60 years) controls to identify potential anti-immunosenescent effects of in vivo antioxidants. The results revealed a significantly higher plasma antioxidant capacity in NONA subjects compared to controls, and these results support a relationship between longevity and intact immune function, which may be underpinned by antioxidant defences which reduce free radical damage to PBMC, thus helping to maintain cell function. The NONA subjects were found to have similar levels of DNA damage in their PBMCs to those found in middle aged controls.