Background <p>Cytapheresis is used to manage inflammatory bowel diseases and immune-related colitis. Two devices, Adacolumn and Immunopure, employ different adsorption mechanisms. However, direct microscopic comparisons of these devices under clinical conditions remain limited.</p> Case presentation <p>We conducted scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to evaluate leukocyte adsorption on carrier beads in Adacolumn and Immunopure. Observations were made before and after treatment in a single case of refractory immune-related colitis. Adacolumn was used in sessions 1–5 and 11, and Immunopure was used in sessions 6–10. SEM revealed that Adacolumn’s cellulose acetate beads exhibited a rough surface, facilitating extensive leukocyte adhesion. Neutrophil pseudopodia were observed penetrating surface irregularities. In contrast, Immunopure’s polyarylate beads displayed a smooth surface with fewer adherent leukocytes, often associated with platelet–leukocyte aggregates. Leukocyte adsorption decreased more markedly in Immunopure over repeated sessions. TEM findings supported these observations, showing distinct cellular interactions with each device.</p> Conclusions <p>This study does not aim to evaluate therapeutic efficacy but rather to provide structural and mechanistic insights into leukocyte adsorption during cytapheresis under clinical conditions. The observed differences indicate that surface morphology plays a critical role in mediating leukocyte interactions, potentially informing optimal device selection and future design strategies. Beyond this single case, the methods and findings in this study may serve as a framework for evaluating adsorption devices in broader inflammatory conditions. Although based on a single case, these findings demonstrate the potential clinical significance and utility of electron microscopic evaluation in understanding device-specific adsorption mechanisms and contributing to the advancement of apheresis technologies.</p>