A large array of heterogeneous aberrations of the immune system have been described in systemic lupus erythematosus (SLE). Since the function and the fate of the immune system cells are governed principally by the biochemical events that follow ligation of specialized cell-surface receptors, we will review in this article recent developments in our understanding of abnormalities in the biochemistry of signals generated either by the antigen-receptor complex or by systems of costimulatory cell-surface molecules, like the CD28/CTLA4:CD80/CD86 and the CD40:CD40L pairs found on the surface membrane of lupus immune cells.