In general it is considered to make the B cell 100-10,000 fold more sensitive to antigen. Kuby quotes an experimental situation in which 10,000 molecules of IgM had to be engaged by antigen for B-cell activation to occur, but only 100 molecules were required when the CD19 coreceptor was activated. So, if the complement system has recognized something as foreign, and the alternative pathway has been activated, B cells will be more sensitive to responding to this antigen. The overall regulatory impact of the B cell coreceptor is that the specific immune system is more likely to respond to a foreign antigen that has activated the alternative pathway than to a self antigen that has not. Some antigens can activate B cells directly with no help from T helper cells.
Responses are antigen-specific.
Role of T cells in B cell activation See Kuby Figure 11-10. Upon antigen interaction, B cells can internalize the antigen, degrade it, combine peptide fragments with class II MHC and present the antigen on the surface. Activated B cells also upregulate surface expression levels of MHC II and of another molecule important for T cell activation, B7. B7-CD28 interactions provide costimulation for T cell activation. Interaction of CD40 on B cells with CD40L on T cells delivers additional activation signals to the B cell.