Chronic allograft rejection has remained a major source of morbidity and mortality following lung transplantation . Survival data from the registry of the International Society for Heart and Lung Transplantation (ISHLT)  demonstrate a significant improvement in the early (up to one year) survival of transplant recipients over the past two decades; however, the rate of decline in survival after the first year is unchanged (figure 1).
The clinical syndrome of chronic lung transplant rejection and the infectious complications related to its treatment with intensified immunosuppression are the major sources of late morbidity and mortality after transplantation .
The clinical aspects and treatment of chronic rejection appearing in the form of bronchiolitis obliterans (BO) and bronchiolitis obliterans syndrome (BOS) are discussed here. Issues related to acute lung transplant rejection, general transplantation immunobiology, and other causes of bronchiolitis are discussed separately. (See "Evaluation and treatment of acute lung transplant rejection" and "Transplantation immunobiology" and "Bronchiolitis in adults".)
Transplanted lungs are susceptible to several different types of rejection.
●Acute cellular rejection – Acute cellular rejection is the predominant type of acute lung allograft rejection and is mediated by T lymphocyte recognition of foreign major histocompatibility complexes (MHC), also known as human leukocyte antigens (HLA), or other antigens in the donor lung [4,5]. (See "Evaluation and treatment of acute lung transplant rejection".)