Kinetics of the decline phase of the primary response to human serum albumin in the bird.

After the intravenous injection of a wide range of doses of human serum albumin (50 micrograms--10 mg) the biosynthesis of antibody maintained constant kinetics although the peak titre increased with the dose of antigen throughout this range. After the rapid rise to a sharp peak at day 7, the antibody levels in the decline period (7-17 days) manifested exponential decay and a constant half-life of 2.2 days which approximated to that observed with passively administered 131I-labelled 7S chicken Ig. It was concluded that after the antibody peak no further antibody production occurs and the switching-off of this antibody response must begin about 3 days before the time of the peak: counts of antibody-containing plasmacytes in the spleen were maximum at day 4. The rate of antibody catabolism and the timing of switch-off of antibody production appeared to be independent of either the dose of antigen injected or the level of antibody at the peak. The mechanisms of the switch-off are discussed.