Complex sentence: The process of negative selection in the thymus guarantees that virtually all T cells have very weak affinity for self-antigens, the study of these lymphocytes in autoimmune diseases and cancer has been difficult until the fairly recent introduction of MHC multimers.
Split into simpler sentences: The process of negative selection in the thymus guarantees that virtually all T cells have very weak affinity for self-antigens. Therefore, the study of these lymphocytes in autoimmune diseases and cancer has been difficult, until the fairly recent introduction of MHC multimers.