Sera and the in vitro induction of immune responses. II. Inhibitory effects of newborn calf sera on primary humoral responses.

The effect of newborn calf (NBC) sera on the in vitro generation of immune responses were studied. NBC sera inhibited the generation of primary humoral responses to SRBC. The inhibitory activity was dose-dependent and diluted out rapidly. Adding inhibitory doses of NBC serum to test cultures at zero or 24 hr resulted in approximately the same amount of inhibition. Pulsing experiments showed that the test cultures were most sensitive to the inhibitory effects of NBC serum during the 24- to 48-hr period. In contrast to its inhibitory effects on primary humoral responses, NBC sera supported the in vitro generation of secondary humoral responses and significantly increased the responses of some of these over control levels. We found that sera obtained from newborn calves before nursing were not inhibitory for primary responses but sera obtained from the same calves 7 days later did inhibit these responses. We postulate that the inhibitory effects of NBC sera may represent a physiologic control mechanism which becomes active when immunologic maturity develops and that it possibly involves the inactivation of cell mediators.