Antigen dependent and independent mechanisms that sustain serum antibody levels.

In this article, we review three mechanisms that contribute to sustaining serum antibody levels: (i) long-lived plasma cells; (ii) persisting or cross-reactive antigen, and (iii) polyclonal activation of all memory B cells. We also present a quantitative analysis of the secondary immune response that reveals the relative contribution of antigen induced short- and long-lived plasma cells versus polyclonal activation of all memory B cells. Antigenic boost sustains high levels of serum antibodies only for a few months, while polyclonal activation sustains low levels of protective antibodies for a human lifetime.