Role of C'3 and Fc receptors in B-lymphocyte activation

Attempts were made to identify the non-Ig lymphocyte receptor responsible for B-cell induction by antigen and polyclonal B-cell activators (PBA). As a first step, the role of C'3 and Fc receptors was analyzed. It was shown that complement could be fixed onto B cells to such an extent that the lymphocytes could not bind complement-coated red cells, but this did not result in induction of polyclonal antibody synthesis, nor did it inhibit the lymphocytes response to PBA. However, the C'3 receptros possessed a passive focussing role in the induction of polyclonal antibody responses. Thus, PBA that had fixed complement activated polyclonal responses at lower concentrations than the same substances that had not fixed complement. Most likely the dual binding of PBA molecules to B cells by the PBA and the C'3 receptors caused more PBA molecules to be bound to each cell. However, the focussing function of the C'3 receptors was several orders of magnitude smaller than that of the Ig receptors. Analogous studies were carried out with Fc receptors. Binding of different types of antigen-antibody complexes did not cause activation of polyclonal or specific antibody synthesis, nor did it significantly interfere with induction of antibody synthesis by PBA substances.