CD81 and Alzheimer disease: The AD group showed a lower CD81 level (median 4.53 ng/ml, interquartile range (IQR) 3.74–6.28 ng/ml vs. median 6.28 ng/ml, IQR 5.35–7.26 ng/ml, Mann-Whitney U = 91.0, P = 0.01) than the HC group (Figure 2A).