In AD, monocyte-derived macrophages infiltrate the CNS via three primary routes: choroid plexus stromal channels (regulated by CCL2-CCR2 chemotaxis), leptomeningeal vasculature (guided by VCAM-1 integrin signaling), and compromised BBB regions (mediated by MMP-9-dependent endothelial remodeling). Here, CCR2 is linked to Alzheimer disease.