Immune reconstitution after hematopoietic stem-cell transplantation and its influence on respiratory infections

Hematopoietic stem-cell transplantation has evolved as the best therapeutic option for many patients with hematologic malignancy, and it is becoming more common as a treatment for metabolic and autoimmune disorders. The conditioning regimen given before transplantation virtually eliminates all pre-existing immunity, and the resulting immune deficits leave the host susceptible to a variety of respiratory infections, many of which carry significant morbidity and mortality. These deficits resolve over time with re-constitution of the immune system. However, the major arms of the immune response mature along different, but predictable, patterns depending on the underlying disorder, the source of stem cells used, and the presence of complications such as graft-versus-host disease. The immune deficit present at specific times posttransplant can assist in determining the most likely organisms to cause infection. This article outlines the recovery of immunity posttransplant, the most common pulmonary infectious complications, the noninfectious complications that mimic infection, and outlines appropriate preventive strategies in this group of markedly immunosuppressed patients.