Plasma cells and their precursors. III. Late phases of anti-bovine serum albumin IgG antibody responses depend upon newly produced IgG B-memory cells.

The sequential appearance of the IgG antibody clones constituting the primary, secondary or tertiary response to BSA was studied in individual rabbits by relating antibody titers, antibody affinities, and clonal spectra obtained by IEF. The results showed that after a single i.v. injection of BSA, the primary response IgG antibodies peaked at day 11 and had a constant and low average affinity during the first 20 days. A slow rise of affinity was observed during the following 20 days. In this period, new IgG antibody clones appeared, though antibody titers decreased. A number of these newly appearing, so-called late-phase clones were isolated by preparative IEF. Their affinities to BSA were high. Secondary responses showed the rapid rise of both titers and antibody affinities typical for the activity of B-memory AFC. One animal immunized for a tertiary response showed a still further increase of antibody affinity in its late phase. These results, together with those described in two earlier papers (1, 2), demonstrate that primary immunization, apart from triggering preexisting IgG AFCP into production of antibodies having low affinity, elicits the origination of IgG (B-memory)-AFCP capable of producing high affinity IgG upon antigenic triggering, and show that some of these may already be triggered during the ongoing response, thereby giving rise to the so-called late phase in the primary IgG antibody response.