B2M and mantle cell lymphoma: Recent evidence strongly suggests that β2M can act as an independent prognostic factor for various types of Hodgkin and non‐Hodgkin lymphomas, including aggressive and indolent lymphomas [54, 55, 56], mantle cell lymphoma [57], extranodal natural killer/T‐cell lymphoma [55, 58], and follicular lymphoma [59, 60].