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Aim: To detect the most common clinical manifestations of Legionella pneumonia (LP) in immunocompromized patients., Subjects and Methods: Clinical manifestations, the results of investigation of bronchoalveolar lavage fluid (BALF) and urine, and the data of lung computed tomography (CT) were studied in patients with blood system diseases and acute respiratory failure (ARF)., Results: The diagnosis of LP was verified in 8 (10.5%) of 76 patients with blood system diseases and ARF. The disease manifested as fever, higher concentrations of inflammatory markers (procalcitonin, fibrinogen), ARF, hypoxemia, and infiltrative lung injury. Six of the 8 patients were switched to mechanical ventilation. Lung CT showed no pathognomonic signs. Five of the 8 patients were observed to have renal dysfunction. The diagnosis of LP was made on the basis of the results of BALF examination in 7 patients and urinary antigen detection in 1. The disease was caused by Legionella pneumophila serogroup 1 in 3 patients and by L. pneumophila of other serogroups in the other patients. Therapy with respiratory fluoroquinolones was performed in 5 patients. Three patients died from progressive ARF and hypoxemia. BALF results were obtained after their death and therapy for legionellosis was not initiated., Conclusion: The incidence of LP is 10.5% in hematology patients. The clinical manifestations of legionellosis are nonspecific; its diagnosis requires bacteriological and/or serological evidence. Due to the high risk of death, it is reasonable to preuse respiratory fluoroquinolones or macrolides in immunocompromized patients with progressive ARF and suspected Legionella pneumonia before diagnosis.

The study of the biological properties of monoclonal antibodies (McAb) to L. pneumophila cytolysin has been carried out. These McAb have been shown to possess no capacity for the in vitro neutralization of cytolysin and the protection of guinea pigs from aerosol infection with L. pneumophila. Still the protective effect of the McAb under study has been observed in experiments with the intraperitoneal infection of guinea pigs, which is indicative of the possibility, in principle, of involving humoral immunity into the protection of the body from Legionella infection.

Acute pneumonias comprise a group of infectious diseases of different etiology which determines many features of clinico-anatomic manifestations of some of their forms including the extension of inflammatory lesions in the lungs. Usually developing due to disorders in the draining function of the bronchi, disorders of the phagocytic activity of leukocytes and alveolar macrophages as well as the presence of immunodeficient conditions, acute pneumonias emerge as complications of other diseases. From 1962 to the present time the rate of their detection in fatal cases increased from 11.6% to 42.8%. Their most frequent causative agents are staphylococci resistant to most antibiotics used for treatment, less frequently Pseudomonas aeruginosa and pathogenic fungi. Also, an increased role in the etiology of pneumonias of conditionally-pathogenic flora (Proteus, Acinetobacter, Enterobacter, Serratia) and frequent pneumonias caused by mixed microflora are observed. Pneumonias were the immediate cause of death in 19.2% of the fatal cases.

The etiological structure of acute pneumonia and acute respiratory diseases was studied with a view to establishing the proportion of L. pneumophila among other causative agents of such diseases. A total of 299 patients were examined over time. The etiological diagnosis based on the data of serological examination was made in 70.6% of the patients with acute pneumonia and in 65% of the patients with acute respiratory viral infections and influenza. In the etiology of pneumonia, the leading role was found to belong to influenza A (H3N2) and B viruses, as well as to adenovirus, while in the etiology of acute respiratory viral infections and influenza, to influenza B virus, adenovirus and Mycoplasma pneumoniae. The importance of L. pneumophila in the etiology of acute pneumonia and acute respiratory diseases was shown. The proportion of L. pneumophila proved to be, on the average, 9.9% in acute pneumonia and 9.8% in acute respiratory diseases. L. pneumophila occurred most frequently in mixed infections in combination with adenovirus and influenza B virus. Diseases of Legionella etiology were found to have a seasonal character, occurring mostly in winter and spring.

A review of the world literature on the "Legionnaires' disease", an infectious disease which was designated in 1979 as "legionellosis" and described in three different clinicoanatomic forms is presented. These forms differ both in manifestations of the disease and in species of the causal agents, legionellae, united in a genus Legionellaceae. Particular attention in the review is paid to morphological lesions observed in legionelloses (macro- and microscopic, and ultrastructural). The necessity of a careful histobacterioscopic examination of tissues in suspected cases of legionellosis and additional crucial bacteriological and serological examinations is noted. Since in the majority of the observations the main pathological process was represented by pneumonia (pleuropneumonia with frequent abscessing), the main differential diagnostic features of legionellosis pneumonia and its differences from other similar infectious diseases are described.