The magnitude and heterogeneity of the immune response to dinitrophenylated bovine gamma globulin was measured in aged and young mice at a cellular level using an inhibition of plaque-forming cell assay. The primary and secondary responses of 24-mo-old mice were markedly depressed in magnitude and restricted in avidity for the DNP determinant when compared to 2-mo-old animals. Bacterial lipopolysaccharide given at the time of immunization increased the restriction in heterogeneity seen in 12- and 24-mo-old mice. Indirect PFCs were more severely depressed than direct PFCs in 24-mo-old mice. Syngeneic, lethally irradiated, 2-mo-old mice reconstituted with aged spleen cells exhibit the depressed and restricted response to DNP-BGG seen in old mice. When 10(8) young thymus cells were given together with old spleen cells the heterogeneity of the response was increased. When 2-mo- and 24-mo-old spleen cells were transferred together into young recipients the magnitude of the response to the young spleen cells markedly reduced. Thus, there appears to be a loss of thymic-helper cells and an increase in suppressor activity in aged animals.

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