APOE and Alzheimer disease: AD risk and immune function have both been affected and influenced by specific genetic variants such as APOE, TREM2 (Triggering Receptor Expressed on Myeloid Cells 2), CD33 (Cluster of Differentiation 33) and MS4A (Membrane-Spanning 4-Domains Subfamily A), the detection of which has emphasized their importance in determining microglial behavior and influencing Aβ processing [59,60,61,62].