Secondary lymphoid organs such as lymph nodes and the spleen constitute central switch points of the immune system. These organs do not only initiate immune responses to pathogens entering the body such as bacteria, viruses or fungi, but they are also essential for the induction of immune tolerance against the body’s own antigens or harmless antigens from the environment. In order to effectively accomplish this task, functional compartments have developed within the lymphoid organs in the course of evolution. The working group headed by Prof. Förster sees to the clarification of the cellular and molecular mechanisms forming the basis of the functional organisation of the lymphoid organs. We are particularly interested in clearing up the function of chemokines and chemokine receptors during the migration of immune cells into the secondary lymphoid organs and within these organs. In the past, the working group managed to gain the fundamental insight that the chemokine receptor CXCR5 is responsible for the positioning of B cells and T cells within the B cell follicles while the chemokine receptor CCR7 plays a pivotal role for the continuous immigration of T cells as well as dendritic cells. In a series of different research projects, we intend to find out to what extent the functional organisation of the lymphoid organs or, respectively, the disorganisation thereof is relevant for the development and maintenance of various disease patterns. The use of two-photon microscopy for in vivo visualisation of the migration properties of these cells is a material requirement for finding an answer to this question.