Studies on Antibody Production Ix. the Cellular Localization of Antigen Molecules (~erritin) in T~ie Secondary Response

The cellular reactions in antibody formation have been studied intensively in recent years. It is now firmly established that antigenic stimulation triggers a typical and specific cellular response in lymph nodes, spleen, and bone marrow, which is characterized by the appearance of hemocytoblasts and the development of immature and mature plasma ceils (1). These cell types are the main contributors to antibody production (2-4). The site of antigen uptake is usually closely associated with the site where antibody-producing cells are formed, but it is clear that not all cells which take up antigen produce antibody. Furthermore, it is not known whether ceils producing antibody contain antigen or antigen fragments although this is postulated by the "instructive" theory of antibody formation. As Humphrey (5) has pointed out, "If we could trace the fate of antigenic molecules from the time of their entry into ceUs until the time when antibody is produced in response to them, we would be on much firmer ground in constructing theories about the mechanism of antibody synthesis." The present experiments on the secondary response in poplitcal rabbit lymph nodes against horse ferritin were designed in the hope of determining whether antigen is present in antibody-producing cells. Ferritin was chosen because it is an antigenic molecule that can be traced under the electron microscope. Ferritin molecules are composed of a highly electron-opaque central core of ferric hydroxide-phosphate miceiles, enclosed by a protein shell (6). In osmium-fixed, methacrylate-embedded tissues only the dense inner cores of ferritin molecules are visible and not the protein (apoferritin) moiety. Antibodies are, however, formed against the antigenic determinants of the protein shell, and are therefore probably identical with those obtained after immunization with the iron-free apoferritin (7). This offers the opportunity to trace by electron microscopy only