Zebrafish have 70% of the same genes as humans and are successfully assisting in the research and study of the development and progression of human diseases, including those affecting the immune system. This project characterises the immune system of an immune deficient zebrafish, generated by mutating a key cytokine signalling receptor component called il2rgc.a. This has involved the characterisation of specific immune cell populations through kidney, spleen, FACS analysis, cell sorting, histology, and microbiome as well as the observation of the immune responses following bacterial or viral infection and the response to cell transplantation from both human and zebrafish donors. This work helps create a platform for further investigation of xenotransplantation of human cancer cells and the correction of the gene defects within human disease. With the il2rgc.a immune deficient zebrafish characterisation and xenotransplantation, we are able to identify human cancer cell growth and migration in real time through fluorescence microscopy, and the use of this information will individualise and pharmacologically treat cancer patients across the globe.