Dysfunction of the immune system in aged individuals includes at least two important factors: accumulation of immunocytes with reduced function and accumulation of lymphocyte clones with self‐reactive potential. Coincidently, there is a profound reduction of the germinal center reaction in the aged. While this reduction is likely the result of age‐associated impairment in lymphocyte function (e.g. diminished response to costimulus, altered lymphokine production etc.). the reduction of germinal centers may itself make an important contribution to further immunological dysfunction.