Multivariable MR with effector memory CD8+ T cell absolute count as a second exposure variable (utilising a Q-statistic minimisation procedure) demonstrated that the causal effect of rheumatoid arthritis was either mediated by immune cells or underpinned by correlated pleiotropy (rheumatoid arthritis effect size: 0.03, 95% CI: −0.007 to 0.055; effector memory CD8+ T cell absolute count effect size 0.07, 95% CI: 0.001 to 0.19). This evidence concerns the gene CD8A and rheumatoid arthritis.