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Increasing evidence implicates HSV type 1 (HSV1) in the pathogenesis of late-onset Alzheimer disease (AD). HSV1 has evolved highly sophisticated strategies to evade host immunosurveillance. One strategy involves encoding a decoy Fcγ receptor (FcγR), which blocks Fc-mediated effector functions, such as Ab-dependent cellular cytotoxicity. Ig γ marker (GM) allotypes, encoded by highly polymorphic IGHG genes on chromosome 14q32, modulate this immunoevasion strategy, and thus may act as effect modifiers of the HSV1-AD association. In this nested case-control human study, 365 closely matched case-control pairs-whose blood was drawn on average 9.6 y before AD diagnosis-were typed for GM alleles by a TaqMan genotyping assay. APOE genotype and a genetic risk score based on nine additional previously known AD risk genes (ABCA7, BIN1, CD33, CLU, CR1, EPHA1, MS4A4E, NECTIN2, and PICALM) were extracted from a genome-wide association study analysis. Antiviral Abs were measured by ELISA. Conditional logistic regression models were applied. The distribution of GM 3/17 genotypes differed significantly between AD cases and controls, with higher frequency of GM 17/17 homozygotes in AD cases as compared with controls (19.8 versus 10.7%, p = 0.001). The GM 17/17 genotype was associated with a 4-fold increased risk of AD (odds ratio 4.142, p < 0.001). In conclusion, the results of this study demonstrate that Ig GM 17/17 genotype contributes to the risk of later AD development, independent of apolipoprotein ε4 genotype and other AD risk genes, and explain, at least in part, why every HSV1-infected person is not equally likely to develop HSV1-associated AD.