The purpose of this study was to investigate changes in immunocompetence associated with aging in humans by the use of various immunologic parameters. Volunteers examined in this study were healthy or apparently healthy without diseases affecting the immune system and ranged from 14 to 89 years of age. The results of this study are summarized in Table I.

The absolute number of peripheral blood lymphocytes (PBL) decreased with aging. The mean count was 2043 in the adult group (less than 60 years of age) and 1659 in the aged group (over 60 years of age). When T lymphocytes were assayed by rosette formation with neuraminidase-treated sheep erythrocytes in the presence of fetal calf serum after 1 hr incubation at 4°C, there was no difference in the proportions of T lymphocytes between the groups, whereas the proportions of T cells as assayed by rosette formation with sheep erythrocytes in the absence of fetal calf serum were significantly depressed in the aged group (p < 0.01). On the other hand, the proportion of T lymphocytes assayed by rosette formation with sheep erythrocytes after centrifugation at 1500 RPM for 5 min at room temperature was higher in the aged group than in the adult group (p < 0.02). The proportion of IgG-Fc receptor-bearing T lymphocytes detected by double rosette formation with sheep erythrocytes and chicken erythrocytes sensitized with rabbit IgG antibody was also higher in the aged than in the adult group (p < 0.05). The proportion of complement receptor-bearing lymphocytes did not change with aging. In addition, the proportion of surface Ig-bearing lymphocytes examined after being treated in pH 4.0 acetate buffer for 1 min to remove nonspecifically bound Ig from the cell surface showed no differences between the two groups.

Response to PHA and concanavalin A was impaired in the aged (aged 70 to 83 years) compared to the young adult group (aged 20 to 40 years) (p < 0.01), whereas the response to pokeweed mitogen was of the same magnitude. PHA response was negatively correlated with the proportion of IgG-Fc receptor-bearing T cells. The mixed lymphocyte culture reaction with mitomycin-treated AL1 cells derived from Burkitt's lymphoma was also impaired in the aged group (p < 0.01) as was cell-mediated cytotoxicity to AL1 cells as target cells (p < 0.05).

Levels of IgG and IgA in plasma were significantly higher in the aged than in the adult group (p < 0.001), whereas those of IgM and IgE were not different. Radioimmunoassay determination of Ig classes revealed that the geometric means of IgG, but not IgM or IgA, synthesized in vitro was significantly increased in the aged compared to the young adult group (p < 0.05) when PBL were cultured in the presence of PWM for 7 days. This increase in IgG synthesis in the aged group was attributed to enhanced T helper activity.

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