Mitochondrial damage is one of the reasons that levels of damaging oxidative compounds rise in the body with age - known as oxidative stress. At the level of molecular machinery, having reactive molecules flying around in large numbers will cause important mechanisms to break down more often. It is generally thought that increased oxidative stress contributes to a range of dysfunctions, and here researchers link it with immune system decline: "Aging is known to affect immune function, a phenomenon known as immunosenescence ... Our study has uncovered several ways in which aging can worsen the body's overall ability to mount an effective immune response. ... All cells generate chemicals called free radicals as a normal part of metabolism. These highly reactive, unstable molecules can readily damage proteins, lipids and other cellular components ... Cells keep 'oxidative stress' in check by producing several enzymes that are scavengers of free radicals. But in aging, increased production of free radicals coupled with cells' decreased production of antioxidant enzymes cause a buildup of damaged proteins and other molecules that can be toxic to cells. The current study is the first to examine whether age-related oxidative stress compromises the function of a type of immune cell called dendritic cells. ... When you are exposed to viruses or bacteria, these cells engulf the pathogens and present them to the immune system ... [Researchers] isolated dendritic cells from aging mice and found that oxidation-damaged proteins had accumulated in those cells and had caused harmful effects. For example, oxidatively modified proteins hampered the function of endosomes, the cell's organelle where pathogens are inactivated. When the mice were injected with a potent antioxidant in the abdominal cavity daily for two weeks, some of the effects of oxidative stress were reversed. This finding has implications for designing vaccines or therapies for humans, especially the elderly, whose weakened immune systems increase their susceptibility to infections and cancer, and reduces vaccine effectiveness. ... Many elderly people respond very poorly to vaccination, so perhaps a cycle of therapy with antioxidants before vaccination might improve their immune response to vaccines."