Although the association between thymic hyperplasia / thymoma and autoimmune myasthenia gravis has been known for some time, the question of causality remains uncertain. Recent research findings indicate, however, that especially in myasthenia patients with thymomas a non-physiological export of naive CD4 + T-cells can take place by the tumour and this could possibly play an important role in the pathogenesis of myasthenia gravis. The investigators want to analyse the functionality and specificity of t-cells generated in thymomas as well as the effect of thymectomy on the immune system.