HLA-C and chronic obstructive pulmonary disease: Given the potential complexities of these interactions, we compared COPD prevalence among the subpopulation with the highest disease risk (HLA-C*07 + KIR2DS1 + HLA-C*12null) versus those with the least risk (HLA-C*12 + HLA-C*07null + KIR2DS1null) and again found strong epistatic (multiplicative) effects among these genetic elements (Figure 2C).