Observations favouring Pneumocystis carinii pneumonia as a primary infection: a monoclonal antibody study on paraffin sections.

Pneumocystis carinii pneumonia is characteristic of immunodeficiency and the organism is probably acquired during early childhood. Since infection is only manifest in the lungs, it has been presumed that the organism lies dormant in these tissues following the primary infection. Conventional staining procedures have, however, failed in the absence of pneumonia to demonstrate consistently any forms of Pneumocystis carinii. To study this problem further, lung sections and hilar lymph nodes from immunodepressed adults with and without Pneumocystis carinii pneumonia as well as lung sections from presumed immunocompetent patients were examined for the cyst and trophozoite forms of Pneumocystis carinii using a monoclonal antibody. The organism was only identified in areas of pneumonia, and the source of the organism in these patients may therefore be a new infection with a different human subtype and not, as previously thought, reactivation of a primary infection.