TREM2 and Alzheimer disease: In the AD brain, TREM2 is expressed by amyloid-associated microglia and peripherally recruited monocytes/macrophages [168,169,170]; upon binding oligomeric Aβ [171], it mediates the myeloid cell recruitment and accumulation around plaques [169,172], as well as amyloid uptake and degradation [173,174] (Figure 2).