Depression is a complex multifactorial condition whose underlying mechanisms have still not been elucidated. Available treatments are therefore far from effective, and it is estimated that 30% of patients are resistant to conventional treatment. Scientists from the Institut Pasteur, in collaboration with teams from Sainte-Anne Hospital, have shed light on the role of brain inflammation as a mechanism that may lead to depression. This is an important discovery in our understanding of the condition, paving the way for new treatment possibilities.