IGHG4 and Alzheimer disease: Proteomic analysis performed on brain supernatants demonstrated that the levels of peptide abundance among the IgG subclasses, IGHG1, IGHG2, IGHG3, and IGHG4, were individually altered in each pathological stage of AD (Kruskal–Wallis test: p = 0.278, p = 0.007, p = 0.120, and p = 0.015, respectively) (Fig. 3A–D).