Age-associated changes in immunity, which are manifested by a reduced immune response and a greater frequency of autoimmune events, are closely linked to the pathology of ageing. Further indications that this is so are the more marked changes in immunity indices in old subjects suffering from cardiovascular, nervous and bone and joint diseases; using other indices, however, such changes appear to be less pronounced. Ageing of the immune system is accompanied by suppression of cellular and humoral elements of older individuals, as demonstrated clearly by experiments involving heterochronic parabiosis and heterochronic transplantation of cells and organs (thymus, spleen and bone marrow). Our findings may suggest that changes in the immune system with ageing are not a consequence of loss or decreases in certain cells or substances; rather, they are due to active suppression of the regulation of cell differentiation and of cooperative interaction between various types of cells. Such effects may be of a systemic or local character and reflect the expression of the organism's ontogenetic programme. In turn, primary disturbances of the immune system can cause several of the secondary ageing phenomena, dominated by various forms of pathology.