RA, linked to MHC shared epitopes and smoking-induced citrullination (Khalid, 2024; Alm et al., 2025; Romero-castillo et al., 2024), involves STAT4 in Th1/Th17 activation and IL-6 in joint destruction (Srivastava and Rasool, 2024; Yanagisawa et al., 2022; Bravo-villagra et al., 2024). Here, HLA-C is linked to rheumatoid arthritis.