Influence of the spleen and thymus on immune responses in ageing mice.

Primary haemagglutinin responses to sheep red cells were depressed in aged C57B1 and (AKR × C57B1)F mice but not in aged C3H mice. Splenectomy depressed haemagglutinin responses in both young and old mice but subcutaneous spleen grafts failed to restore immune responsiveness of splenectomized mice to normal levels. Multiple spleen or thymus grafts in C57B1 mice failed to prevent the age-related decline in responsiveness to sheep red cells as estimated either by the haemolytic plaque-forming cell response in the spleen or haemagglutinin titrations. Multiple spleen and thymus grafts also failed to improve pre-existing depressed responses in old age. Although adult thymectomy causes a delayed depression in responsiveness to sheep red cells, depressed thymus function in old age does not appear to be a factor limiting immune responses to sheep red cells in aged mice.