28 including eosinophils, and may inhibit eosinophil trafficking to the intestinal mucosa. This led us to hypothesise that VEDO induction therapy may influence gut mucosal eosinophil abundance as well as serum eotaxin-1 levels, which might therefore be used as potential biomarkers for response to therapy. PART V: COVID-19 AND IBD In the final part of this thesis, we focus on COVID-19 in the context of IBD and the scope of this thesis. In Chapter 27, we present a comprehensive overview of COVID-19 pathophysiology and the role of the ACE2 protein. Since patients with IBD often use immunosuppressive drugs and may have uncontrolled disease activity, they are at increased risk of viral infections. In Chapter 28, we present a case series of individuals admitted to Dutch hospitals who contracted COVID-19 but clinically presented with symptoms that typically occur in IBD, either being a first presentation of the disease or a flare of pre-existing IBD. Here, we share our clinical experience with these first patients and propose suitable management strategies with the goal to initiate the discussion on the efficacy and safety of IBD treatments in the context of COVID-19. Since oxidative stress emerged as a key pathophysiological process in COVID-19, we set out to investigate the potential of serum free thiols in individuals with COVID-19, which is presented in Chapter 29. In 2021, vaccination campaigns were globally initiated, but there was uncertainty about seroconversion rates and, thus, the adequacy of the serological immune response against SARS-CoV-2 upon vaccination in patients with IBD. In Chapter 30, we present a study where we investigated the effects of different types of IBD medication on the magnitude of the serological antibody response upon vaccination in patients with IBD. In Chapter 31, I summarize the main findings of this thesis and discuss their mutual relations and implications for clinical practice. In addition, I will propose several suggestions for future research and outline future perspectives. Chapter 1
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