The recent demonstration that plasma C3 levels are elevated and a potential blood biomarker of CAA in MCI patients suggests a role for complement [95]; indeed, an IHC study of CAA in AD brain showed deposition of C3 fragments and C9 in the amyloid-laden vessels that correlated with severity of CAA [96], while others report staining for C1q, C3d, C6 and C5b-9 in amyloid-positive cerebral vessels from CAA cases [91]. Here, C3 is linked to Alzheimer disease.