CD74 and red color blindness: The results of our study suggest that (1) serum anti-CD74 IgG and IgA antibodies are elevated in patients with AS compared to healthy controls, (2) serum anti-CD74 IgG antibodies are not elevated in patients with early axSpA compared to patients with CBP, (3) serum anti-CD74 IgA antibody levels are elevated in patients with early axSpA compared to patients with CBP, but (4) serum anti-CD74 IgA antibodies have no diagnostic value in patients with early back pain because of small numerical differences.