Rabbits were immunized to two antigens and 18–55 days later exchange transfusion was performed using blood of rabbits immunized to one antigen only. By this means, serum antibody levels to one antigen were reduced 50–84% while maintaining serum antibody levels to the second antigen. After exchange, serum antibody levels of the removed antibody rose rapidly for 24–48 hr and then more slowly, reaching peak titers an average of 8 days later. The peak titer was 48–222% higher than the preexchange titer. The specificity of this rebound excluded as a cause nonspecific changes in Ig levels. Passive administration of antibody to a third antigen 4–7 days before the exchange indicated that re-equilibration of preformed antibody was not a major factor in the rebound. A change in the ratio of IgM to IgG antibodies as a cause of an increased neutralization titer in the postexchange sera was also excluded. It was therefore suggested that a change in the rate of antibody formation had occurred, although other changes in the quality of serum antibody were not excluded.

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