Memory B Cells but not Virgin B Cells are Activated in Germinal Centers

Abstract

In chimaeras constructed by transferring K-allotype distinct DNP-haemocyanin-primed thoracic duct lymphocytes (TDL) into irradiated (hind limb bone marrow shielded) recipients the activation of virgin (host allotype) B cells is often transient falling to negligible levels within 14 days (1). The lower affinity of antigen receptors on virgin B cells compared to memory B cells precludes their continued activation as antigen becomes limiting. However, as antibody levels rise antigen is re-localised into follicles in the form of immune complex (2,3). Recently it has been shown that virgin B cells after transfer do not migrate into follicular structures, in contrast to transferred TDL (4). This raises the possibility that virgin and memory B cell activation occurs at different sites. When the supply of antigen is high (soon after immunisation) both virgin and memory B cells are activated at extra-foilicular sites but as antigen levels fall and re-localisation onto follicular dendritic cells (FDC) occurs, only memory B cells gain access to the antigen supply.