Introduction: Ankylosing spondylitis is also known as radiographic axial spondyloarthritis, a rare genetic disease affecting people with hereditary factors.  In addition, it is one of the autoimmune diseases with systemic chronic inflammatory, progressive, immune-mediated reactions. It may be classified as seronegative spondyloarthropathy, which tests negative for rheumatoid factor and antinuclear antibody. Treatment of ankylosing spondylitis includes lifestyle modification and use of drugs such as the biologic agent infliximab or its biosimilar, CT-P13 infliximab. Despite their therapeutic usefulness, these agents are associated with a number of serious adverse effects such as immunogenicity. Methods: A retrospective open-label study was conducted from December 2021 to March 2022 at the Rheumatology Unit, Baghdad Teaching Hospital, Medical City, Baghdad. Forty-four patients were taking Infliximab, and another 50 patients were taking CT-P13 (Remsima), both at a dose of 5 mg/kg for 3 months prior to recruitment in current study. Disease activity was assessed by ankylosing spondylitis disease activity (ASDAS-CRP) score, while antibodies and C-reactive protein were tested using ELISA technique. Results: Immunogenicity of the biosimilar CT-P13 infliximab was higher than that of the reference infliximab (P< 0.05). In addition, a number of patients in both treatment groups developed hypersensitivity reaction to either drug. However, there was no statistically significant correlation between the two variables (P> 0.05). Conclusion: Immunogenicity of infliximab or its biosimilar (CT - P13) may result in reduced therapeutic effectiveness manifested as increased disease activity. Also, such immunogenicity may be triggered by previous biological treatment and/or the total number of doses.