This dissertation examines the clinical relevance of drug immunogenicity. The author conducted a systematic review and meta-analysis of 17 studies involving 936 patients. The analysis found that anti-drug antibodies (ADAb) significantly reduce therapeutic responses by around 80% and concomitant immunosuppression decreases ADAb production by 64%. Nearly half of ADAb-positive patients developed infusion-related adverse events. A bridging ELISA assay was found to be a practical method for routinely assessing ADAb. A new algorithm was proposed integrating immunogenicity testing, and when applied in a clinical trial it led to better outcomes than the standard approach, with ADAb-positive patients having higher disease activity. The dissertation concludes ADAb have an important clinical impact.