[Pneumonia in the immune compromised host]

The term "immunocompromised host" is generally applied to a variety of patients with different host defense defects. Pulmonary disease in the immunocompromised host remains a major cause of morbidity and has a high mortality. During the initial evaluation of the patient, it is helpful to define which of the three arms of the host defense system is most likely to be affected. Impaired granulocyte function, as seen after chemotherapy, predisposes to bacterial and fungal infections. Deficiencies in the humoral immune system predisposes to infection with encapsulated organisms, such as Streptococcus pneumoniae and Haemophilus influenzae. Impairment of the cellular immunity is a special problem of the transplant patient. Besides bacteria, a number of unusual microorganisms, such as viruses (cytomegalovirus, varicella-zoster virus, herpes simplex virus), protozoa (Toxoplasma gondii) and fungi (Pneumocystis carinii, Aspergillus, Cryptococcus neoformans) have to be considered in this group of patients. The work-up usually requires an invasive technique, such as a bronchoalveolar lavage or lung biopsy to establish the diagnosis. The initial therapy of a patient with a pulmonary infiltrate often involves an empiric broad-spectrum antibiotic therapy. Whether an additional treatment against atypical bacterial pathogens, fungi or viruses should be started, depends on the clinical presentation, the underlying type and duration of immunosuppression and the radiographic evolution of the infiltrate.