Recent evidence suggests that blood monocytes are associated with familiar and sporadic forms of AD and PD, and CD33 and TREM2 gene variants have been identified as risk factors of AD associated with PD and coupled with myeloid cells, which infiltrate the degenerated brain, while the clearance of vascular Aβ is carried out by patrolling monocytes. This evidence concerns the gene CD33 and Alzheimer disease.