Several mechanisms have been postulated to explain T-cell large granular lymphocytes clonal proliferation after solid organ and haematopoietic stem cell transplantation, as summarized. (Abbreviations: APC, antigen presenting cell; HSCT, haematopoietic stem cell transplantation; SOT, solid organ transplantation; LGL, LGLL, large granular lymphocytes; GvHD, graft versus host disease).