C5 and Alzheimer disease: Furthermore, a large group of proteins had lower levels in AD, including immune system-related complement factors (C2, C3, C5, C6, factor-B [CFB], factor-H [CFH]) and factors that interact with the complement system (plasminogen [PLG], prothrombin [F2], serum amyloid P component [APCS], and C-reactive protein [CRP]).