T-cells are the immune system’s security force. They seek out pathogens and rogue cells in the body and put them out of action. Their precursors are formed in the bone marrow and migrate from there into the thymus. Here, they mature and differentiate to perform a variety of tasks. Scientists at the Max Planck Institute of Immunobiology and Epigenetics in Freiburg have now succeeded for the first time in observing the maturation of immune cells in live zebrafish embryos. During their development, the immune cells migrate into and out of the thymus more than once. “we can now carry out direct tests to determine what effect certain substances have on the formation and maturation of the T-cells and the thymus tissue,” says Thomas Boehm, who lead the study. Therefore, the study not only contributes towards a better understanding of the way the immune system works, the method could also help with the development of drugs to treat malfunctions of the thymus.