Immune system alterations during ageing are complex and pleiotropic, suggestive of remodelling or altered regulation, rather than simple immune deficiency. The most dramatic changes with age occur within the T cell compartment, the arm of the immune system that protects against pathogens and tumours, consistent with the increased incidence and severity of infection and cancer in the elderly. Indeed, autopsy studies confirm infection as the major cause of death in the very old. Increased serum levels of inflammatory mediators are another hallmark of ageing, suggestive of either regulatory defects or an ongoing attack on sub-clinical neoplastic disease or infection. Qualitative changes in antibody production, including those secreted by the gut mucosal immune compartment, affect responses to foreign antigens as well as to prophylactic vaccines. Innate immunity, the first line of defence that precedes the antigen-specific T and B cell responses, also undergoes changes with age. Some of the immune effects associated with ageing are secondary to overall organismic changes, such as alterations in the viscosity of cell membranes and proteolytic cellular machinery. Evidence suggesting that immune system changes may be involved in some major age-related pathologies, such as atherosclerosis and Alzheimer's disease, will be discussed.