Also, the association between increased intrathecal C3 in other neurodegenerative diseases [17] and our findings here with the highest CSF C3 levels in patients with more MRI lesions and in the progressive phases of disease points to potentially important pathways involved in the chronic neurodegenerative phase of MS, which is less understood than its initial relapsing-remitting phase. Here, C3 is linked to myeloid sarcoma.