C5 and Osteopenia: Thus, the combined used of standard therapies and of inhibitors of C5a-C5aR1 axis might be a successful strategy for the treatment of bone pathologies in which inflammation and complement system are known to be crucially involved, as rheumatoid arthritis in particular, for which also clinical trials have been conducted using C5aR1 inhibitors, but also for osteopenia and osteoporosis, fracture healing and metastatic bone disease.