GFAP and Alzheimer disease: Using data from two National Institute on Aging cohorts, the current study examined the association of plasma biomarkers of AD pathology (Aβ42/40, pTau181), neuronal injury (NfL), and reactive astrogliosis (GFAP) with state‐of‐the art MRI measures of myelin content (MWF) and axonal density (NDI) across a wide age range.