TREM2 and Alzheimer disease: Down-regulation of the triggering receptor enriched in myeloid/microglial cells (TREM2), a transmembrane glycoprotein enriched in the brain’s microglial ‘immune surveillance’ cells similarly appears to play a role in tau pathology and the inability of brain cells in AD to efficiently phagocytose pathological proteins that include amyloid peptides [11,12,25] (Figure 3).