TNF and rheumatoid arthritis: As a consequence, high serum levels of ACPA-IgG with high avidity in RA patients can account for the seemingly paradoxical reports that high serum levels of ACPA-IgG predicted unresponsiveness to TNF-blocking therapy [17] and, at the same time, that decreases in ACPA-IgG levels are associated with response to TNF-blocking therapy [16,38].