TREM2 and Alzheimer disease: 1. TREM2 activation levels in the microglia of patients with MCI could indicate AD trajectory. 2. Activated monocytes may serve as a biomarker for microglial TREM2 AD activation since they correlate with TREM2 status in the microglia. 3. TREM2 activation in AD may lead to immune response, anti-apoptotic signaling, and cytoskeletal changes in the microglia.