[Histopathological diagnosis of opportunistic infection].

The histopathological diagnosis of infectious diseases has definite and inevitable roles not only in identifying the appropriate treatment for the infected patient but also in protecting society from unnecessary risks of pathogen transmission and from exposure to biohazards. The potential for opportunistic infection has been increasing, in response to the gradual increase of aged and immunocompromised patients. Globalization of the world has promoted importation of non-domestic infectious diseases. This article reviews the following points. These include 1) the modes of defense mechanisms against pathogens (neutrophils vs. T-lymphocytes vs. antibodies), 2) epidemiologic aspects of opportunistic infection, including intrinsic, exogenous and imported routes of infection, 3) histopathological features of bacterial pneumonia, 4) immunohistochemical demonstration of pathogens on routine paraffin sections, using commercially available antibodies or patient sera, 5) opportunistic infection in immunosuppressive states, such as AIDS, hematopoietic stem cell transplantation and other special conditions, and 6) the pathologists' function in the prevention of nosocomial (hospital-acquired) infection with MRSA and tuberculous bacilli. Pathologists must realize the importance of correct and prompt histopathological diagnosis of a wide variety of infectious diseases in both immunocompetent and immunocompromised patients.