Immunosenescence is the consequence of the continuous attrition caused by chronic antigenic stress. The most important characteristics of immunosenescence (accumulation of memory and effector T cells, reduction of naive T cells, shrinkage of T cell repertoire, reduction of the immunological space) are compatible with this assumption. Immunosenescence can be taken as proof that the beneficial effects of the immune system, devoted to the neutralization of harmful agents early in life, become detrimental late in life, in a period not foreseen by evolution. This perspective could explain the mechanisms of the ageing process as well as the pathogenesis of age-related diseases.