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Nosocomial pneumonia is frequent after haemorrhage and trauma, and often contributes to multiple organ system failure, morbidity and mortality in this setting. Although the percentages and numbers of bacterial polysaccharide antigen-specific pulmonary B cell clonal precursors arc markedly decreased after haemorrhage, the effects of haemorrhage on pulmonary plasma cells actually producing antibody to these antigens are unknown. To investigate this question, the numbers of intraparenchymal pulmonary plasma cells producing antibody against the bacterial polysaccharide antigen levan (from Aerobacter levanicum) as well as bacterial antigen specific secretory IgA (sIgA) titres in the lungs were determined at various time points after 30% blood volume haemorrhage. Reduced numbers of bacterial antigen specific pulmonary plasma cells were found for more than 21 days following haemorrhage. An almost complete disappearance from the lungs of levan specific plasma cells occurred between 3 and 21 days after blood loss. Titres of bacterial antigen specific sIgA in the lungs were decreased starting at 3 days post-haemorrhage and remained significantly depressed for more than 35 days after blood loss. These results demonstrate that haemorrhage produces profound and long-lasting suppression in bacterial antigen-specific pulmonary plasma cell function. Because these effects do not occur immediately post-haemorrhage, immunization techniques able to enhance bacterial antigen specific sIgA titres at pulmonary surfaces may be able to increase resistance to nosocomial pneumonia if administered shortly after injury and blood loss. [ABSTRACT FROM AUTHOR]