CD200 and Alzheimer disease: Once AR@ENV crosses the BBB, it preferentially accumulates in neurons rather than in microglia (Fig. 5), likely due to specific membrane protein interactions (e.g., CX3CL1-CX3CR1 and CD200-CD200R).40 This selective targeting echoes findings from prior studies showing that exosome-like vesicles exhibit neuron-specific tropism in AD models,49 although the full mechanism remains to be elucidated.