C4A and rheumatoid arthritis: For the classification of RA, the AUC of the random forest model reached 0.9949 (95% confidence interval [CI] = 0.9792–1) (Figure 6A), and the top 15 dominant antigens in the model were SAA2, C-reactive protein (CRP), leucine-rich alpha-2-glycoprotein, fibrinogen alpha chain, annexin A1, complement component C9, complement C4-A, SAA1, carbonic anhydrase, testicular tissue protein Li 70, ficolin-3, ACX136, hemoglobin subunit alpha, paired like homeobox 2B, and beta-actin-like protein 2 (Figure 6A).