Question
Inclusion bodies.
Answer
Inclusion bodies are distinct, microscopically visible structures that form within infected cells during certain viral (and some other) infections, representing accumulations of viral particles, viral proteins/nucleic acid, or altered host cell components — their characteristic location, staining, and morphology can provide important diagnostic clues to the underlying infection.
Classification by location:
- Intranuclear inclusions — e.g., Cowdry type A bodies in herpes simplex virus and varicella-zoster virus infection (eosinophilic, surrounded by a clear halo, displacing nuclear chromatin to the periphery); owl’s-eye inclusions in cytomegalovirus infection (large, single, eosinophilic intranuclear inclusion with a surrounding halo, in an enlarged cell).
- Intracytoplasmic inclusions — e.g., Negri bodies in rabies (eosinophilic, round-to-oval cytoplasmic inclusions in infected neurons, classically in the hippocampal Purkinje/pyramidal cells, historically the standard post-mortem diagnostic test for rabies); Guarnieri bodies in vaccinia/variola (smallpox) infection; Bollinger bodies in fowlpox.
- Both nuclear and cytoplasmic inclusions — e.g., measles virus infection.
Clinical/diagnostic significance: histopathological demonstration of a characteristic inclusion body provides rapid presumptive/confirmatory diagnosis for certain viral infections, particularly historically important where culture/molecular methods were unavailable — Negri bodies for rabies remain a classical example taught for their diagnostic and historical significance, though modern diagnosis now relies more on immunofluorescence and PCR-based methods. In some cases (e.g., cytomegalovirus, herpesviruses), demonstration of the characteristic inclusion in a tissue biopsy remains a useful adjunct to molecular/serological diagnosis.