Furthermore, while lack of CD46 co‐stimulation during T‐cell activation (in patients with C3 or CD46 deficiency) leads to severely reduced Th1 responses and recurrent infections 10, 11, 13, pathologically increased intracellular C3 activation and autocrine CD46 simulation is connected with the hyperactive Th1 responses present in rheumatoid arthritis (RA) 14. Here, CD46 is linked to rheumatoid arthritis.