Age-related decline of immune system not only increases susceptibility of infection, but also delays of wound healing. Although the number of innate immune cells such as neutro- phils and macrophages does not decline by aging, the functions of these cells such as phagocy- tosis, migration toward infection sites decline by aging. Production of inflammatory cytokines by microglia, one of tissue macrophages increases by aging, which causes Alzheimer disease. Human thymic epithelium, which harbor lymphocytes in thymus for development, show a continuous involution from the first year, which replaced by adipose tissue. Age-related involu- tion of thymic epithelium results in the decrease of nalve T cells. Chronic cytomegalovirus infection induces clonally-expanded CD8 T cells and perturbations in the peripheral T cell repertoire.