Researchers in Regensburg have investigated an important part of the immune system’s self-control: the so-called regulatory T cells. These cells can be either ‘police’ or ‘mechanics’. The ‘T cell police’ monitor other immune cells and ensure that they are less active. The ‘mechanic T cells’ (regulatory T cells found in the tissues) help repair damaged tissue in organs. The team in Regensburg discovered that a specific protein in the T cells is partly responsible for whether a T cell becomes a policeman or a mechanic. The protein is called rbpj (recombination signal binding protein for immunoglobulin kappa J region). Without rbpj, the T cells become mechanics and lose their ‘policing capabilities’. They can no longer control or switch off other immune cells. In the study, the team of scientists switched off rbpj in many ‘mechanic cells’, but not in any ‘police cells’. An unregulated immune reaction was then observed during the test in which lymph nodes became swollen with reactive germinal centres. This kind of reactive germinal centre is only found in an immune reaction. Cells mature in this germinal centre that form antibodies (B cells), which are used in the immune defence. The researchers performed the study on mice. The scientists in Regensburg are now further investigating exactly how these ‘mechanics’ come into being and how the findings can be applied to treatments. A balanced immune system is important for good health. If it carries on unchecked, allergies can develop; in the worst case, the immune system can destroy the body’s own tissues (autoimmunity). On the other hand, if the immune system does not work well, the body will be more vulnerable to infectious diseases and tumours. Fortunately the immune system usually finds the right balance by itself. Nature Communications 2019