In order to study the localization pattern of specific antibody-containing cells and extracellular antibody in the spleen during a primary immune response, the antigen human serum albumin (HSA) was injected intravenously in rabbits, either free in solution, or associated with the surfaces of liposomes as a model of membrane-associated antigens. Demonstration of specific antibody-containing cells was performed by incubation of sections of spleen with HSA-HRP conjugates, followed by peroxidase cytochemistry.
Specific anti-HSA antibody-containing cells were detected already at 4 days after injection of the antigen. The bulk of these cells localized initially in the outer parts of the periarteriolar lymphocyte sheaths (PALS) and around the terminal arteriolar branches. Both extracellular antibody and specific antibody-containing cells were also found in the follicles of the spleen. Arguments are given that extracellular antibody precedes the development of specific antibody-containing cells in the follicle. This extracellular antibody probably represents antigen-antibody complexes trapped in the follicles as soon as the antigen in the circulation is complexed by the first antibodies produced during the immune response. The localization pattern of specific antibody-containing cells and extracellular antibody did not differ markedly when rabbits injected with free or liposome-associated antigen were compared. Results are discussed particularly with respect to the role of follicles in the immune response.