Age-related immune dysfunction contributes to the vulnerability of old individuals to infection, e.g., animal model studies demonstrate the association between age-related decline in T cell-dependent immunologic responses and the decline in resistance against viral, bacterial, and parasitic infections. This review briefly describes age-related changes in the immune system at the systemic, tissue and cellular levels. At the systemic level, emphasis is on polymorphic effects of aging; at the tissue level, emphasis is on the vulnerability of primary tissues engaged in the generation of antigen-responsive cells and on the difference in the onset and rate of changes between different peripheral tissues of the system; and at the cellular level, emphasis is on the qualitative changes at the surface receptor, cytoplasmic and nuclear levels.