Relatively little is known about the antigenic structure of Pneumocystis carinii and the immunopathogenesis of pneumonitis caused by P. carinii. To begin to define the antigenic character of the surface of this organism, we have produced murine monoclonal antibodies that react with the surface of P. carinii (obtained from rats), as detected by immunofluorescence and immunoelectron microscopy. Immunoblot analysis revealed that the six antibodies described in this report bound an antigen with an apparent molecular mass of 90,000-95,000 daltons. Although all six monoclonal antibodies bound P. carinii obtained from rats, only one (5E12) was also able to bind P. carinii obtained from rabbits, ferrets, and a human; this result demonstrated that isolates of P. carinii obtained from different species are not antigenically identical.

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