Takayasu's arteritis (TA) is a large vessel vasculitis of unknown origin involving the aorta and its major branches. T cell mediated autoimmunity is thought to play a major role in its pathogenesis. Based on findings in one patient with highly active disease we analysed for the first time the B cell compartment of patients with TA and could show that marked expansion of plasma cells can be found that correlates with disease activity and therefore qualifies as a novel biomarker. In addition treatment with the B cell depleting agent Rituximab proved to be successful in refractory patients. Therefore the role of B cells in TA needs to be reconsidered.