Antibody production in mice: II. The mechanism of antigenic stimulation in the secondary immune response

To study the mechanism of antigenic stimulation in the secondary immune response, primed lymphoid cells were stimulated with antigen in various ways in vitro. The effect of antigenic stimulation was assessed by the antibody titres obtained after in vivo culture of primed cells in X-irradiated recipients.

Primed cells were much more effectively stimulated by antigen—antibody (Ag—Ab) complex than by free antigen. Primed cells could also be stimulated by spleen or lymph node cells from normal mice which had been exposed to free antigen or Ag—Ab complex in vitro or in vivo and thoroughly washed. Under these conditions, Ag—Ab complex was again much more effective than free antigen. When the cells were incubated with Ag—Ab complex, the dose of antigen bound to the cells was somewhat increased. But this increased binding of antigen could not solely account for the increase in immunogenicity.

It is suggested that the ingestion of antigen by macrophages is facilitated by the presence of antibody and that the macrophages mediate the effective immune stimulus to memory cells.

The effect of antibody in increasing the immunogenicity of antigen was lost completely when antibody was digested with pepsin. Thus, the Fc portion of antibody seemed to be important for this effect. However, it was demonstrated that antibody does not operate by becoming attached to macrophages as cytophilic antibody, and that complement is not involved in this process. The augmenting mechanism of antibody on the antigenic stimulation mediated by macrophages was discussed.