CD40 and systemic lupus erythematosus: B cell activation, plasma cell differentiation, antibody production, and isotype-switching are all humoral immune responses that cannot be activated without the co-stimulatory molecule CD40 binding to its ligand CD40L.257 More CD40L-expressing peripheral macrophages were seen in SLE patients than healthy controls.258 Consistent with this discovery, recombinant CD40L greatly enhanced the synthesis of total IgG by SLE B cells but not normal B cells.259 However, SLE patients and normal control B cells exhibited equivalent CD40 expression levels.