Liver transplantation is a life-saving therapy that can drastically improve the quality of life in children with acute or chronic liver illness. Even though the long-term survival rate has risen, some patients develop immune system dysregulation. The most common is liver transplantationassociated food allergy. However, the immunological mechanism leading toward the food allergy is still unknown. The main aim of the project was to study how T lymphocytes develop before, during and until 5 years after liver transplantation. Flow cytometry was used to characterize T cell subsets in liver transplanted children and the proportion of different T cell subsets was analyzed at different time points following liver transplantation. Serum cytokine levels in liver transplanted children who developed food allergy were analyzed by multiplex bead-based immunoassay and compared with cytokine levels of liver transplanted children without food allergy with age and gender-matched healthy controls and food allergic controls with no history of liver transplantation. Univariate analysis showed that children that had been transplanted had a significantly increased proportion of CD62LnegCD45RAneg effector CD4+ T cells 4-5 years after transplantation when compared to before their transplantation. A linear mixed-effect model showed an overall significant increase in the percentage of CD62Lneg CD45RAneg effector CD4+ T cells and CD45RAneg CD62L+ central memory T cells. In contrast, there was a significant decrease in naïve T cells and TEMRA of CD4+ T cells. OPLS-DA analysis showed that there was a positive association between being liver transplanted and having activated T cell phenotypes, i.e., higher proportions of TEMRA CD8+ T cells, effector CD4+T and CD8+ T cells and higher serum cytokines levels. Multiple comparisons between groups showed that liver transplanted children without food allergies had higher concentrations of IL-6 when compared to liver transplanted children with food allergies. Also, liver transplanted children with food allergies had higher levels of cytokines IL-10, IL-6, IL-12p70 when compared to healthy and allergic controls. Thus, children that have gone through liver transplantation have an immune phenotype that is associated with T- cell activation.