Immunogenicity refers to development of antibodies by the adaptive immune system in response to foreign substances. The development of anti-drug antibodies were extensively studied in rheumatoid arthritis and it was shown that anti-drug antibodies has a varying impact on the clinical efficacy depending on whether these antibodies are neutralizing or non-neutralizing. Recent review demonstrated that neutralizing antibodies are associated with a reduced chance of achieving a minimal disease activity or clinical remission, decreased drug survival, increased dose escalation and adverse drug reactions in RA patients. However data regarding the immunogenicity in patients treated with AS is scarce and somewhat controversial. In a small study including 38 AS patients treated with infliximab de Vries et al showed that anti-infliximab antibodies was found significantly more often (59% vs 5%) and mean serum through infliximab levels were significantly lower in ASAS20 non-responders. In the above-mentioned study infusion reactions were also seemed to be associated with the development of anti-drug antibodies. In another study de Vries et al (26) were found no antibodies to etanercept and similar serum etanercept levels in responder and non-responder AS patients (n=53). Same group also observed that anti-adalimumab antibodies were become detectable in 31% of AS (n=35) patients in 6 months period and this corresponded with diminished serum drug levels.