CD74 and red color blindness: In a second study of patients with early axSpA (n = 274) and with non-SpA chronic back pain (CBP) (n = 319), 46.4% of axSpA patients and 47.9% of CBP controls had IgG antibodies to CD74 while 54.7% of axSpA patients and 37% of CBP controls had IgA antibodies to CD74 (9).