Our immune system plays a role in eliminating microbes and cancer cells, but also functions as a sentinel, preventing the emergence of defective white blood cells which could attack our own tissues and, therefore, trigger an autoimmune disease. However, every day patients are diagnosed with autoimmune diseases and nobody really understands why immune safeguard mechanisms have failed. We have described a new factor named BAFF/BLyS, which acts on a subset of immune cells and promotes their survival. Overproduction of this factor leads to the inappropriate survival of harmful immune cells and tissue destruction by these cells. Importantly, the serum of patients suffering from autoimmune conditions contains high levels of BAFF. We have collected evidence suggesting that subsets of white blood cells might have an adverse role in the progression of BAFF-led autoimmune disorders and inflammation. In this project, we would like to identify harmful white blood cells, how they abnormally develop and molecules specific for these cells to be used as targets for therapeutic intervention in various autoimmune diseases. The aim of this work is to find ways to refine treatments of autoimmune diseases by targeting pathogenic cells while preserving the healthy side of our immune system.