Solid organ transplantation (SOT) has become a commonly performed procedure for patients with end-organ disease. Many SOT patients' way of life is associated to the same community exposure to influenza as the general population. However, mostly because of the lifelong immunosuppressive regimen, SOT patients are at increased risk for influenza-associated morbidity and mortality. This is further complicated by the reduced vaccine immunogenicity among SOT patients, making the only efficient preventive strategy suboptimal. In this context, there is a need to thoroughly evaluate immune responses to natural influenza infection and influenza immunization in SOT patients. This will improve clinical management by detecting patients most at-risk for severe influenza, but also optimize prevention by improving vaccine immunogenicity. The line of research described in this work reflects an ongoing attempt to better understand the humoral and cellular immune responses against influenza natural infection and immunization in SOT patients, which immunological and clinical factors may be associated with protection against (re)infection and whether some biomarkers of disease severity can be identified