To detect foreign invaders and to communicate with other cells of the immune system, B lymphocytes carry a multitude of receptor proteins on their surface. In the past, it was thought that most of these receptors are randomly distributed on the cell surface and only become organized upon lymphocyte activation. Recent studies showed, however, that many of these surface proteins are pre-organized in nanoscale protein islands (here called nano-islands) with a size of 50-150 nanometer (nm). We have developed a Fab-proximity ligation assay (Fab-PLA) which allows us to study the organization of membrane proteins at 10-20 nm distances. With this method, we discovered that the B cell antigen receptor (BCR) of the classes IgM and IgD are located in different class-specific nano-islands.Due to the technical limitations of classical biochemistry (detergent lysis), light microscopy (diffraction limit of 250 nm) and electron microscopy (fixation artifacts), the nanoscale organization of membrane proteins is not well studied. With the Fab-PLA method, we can, for the first time, explore this unknown 200-20 nm space and analyze the composition and stability of the different nano-islands on the B cell surface. We also will develop new methods such as proximity biotinylation and proximity proteomics to better study these nanostructures and learn more about their role in B lymphocyte activation and human diseases such as lymphomas and autoimmune diseases.Although more than 50% of all drugs target membrane proteins, the nanoscale organization of these proteins is poorly characterized. To learn more about the nano-islands organization of membrane proteins is thus of utmost importance for a better understanding of the action of these drug treatments and to improve them. Our study may also provide better markers for specific disease stages and could lead to new drugs influencing nanoscale membrane processes.