Figure 2. Epistatic interaction between the MHC and RCAα block in Ro/La autoantibody-positive patients with primary Sjögren syndrome (pSS). (A) Odds ratios (y-axis, logarithmic scale) derived by logistic regression for the cross-classification of HLA DR3, DR15 and RCAα AH1, AH3 genotypic combinations (dominant coding) in Ro/La autoantibody-positive patients with pSS (n = 92) relative to controls (n = 98). The vertical bars represent 95% confidence intervals, and the horizontal line represents an odds ratio of 1 (no effect). HLA DR3, DR15 alleles and the RCAα AH3 haplotype were independent risk factors for autoantibody-positive pSS (ie, multiplicative risks), but there was an epistatic interaction between HLA DR3 and RCAα AH1 (interaction term p = 0.021). The genotypic combination of HLA DR3 and RCAα AH1 was the greatest genetic risk factor for autoantibody-positive pSS (OR 15.7, p = 10−8), but in the absence of DR3, there was no effect of RCAα AH1. (B) Pie chart depicting the relative proportions of risk genotypes. The HLA DR3–RCAα AH1 epistatic combination was present in 48% of autoantibody-positive patients with pSS compared with 8% of controls. The majority of other patients with pSS carried any combination of HLA DR3, DR15 and RCAα AH3.