Which fixative is commonly used as a primary fixative in IHC?

• A. Bouin's solution
• B. 10% neutral buffered formalin (NBF)
• C. Acetone
• D. Ethanol

Answer: (B)

The main idea here is which fixative is used first to preserve tissue for IHC in routine practice. 10% neutral buffered formalin is the standard primary fixative for most immunohistochemistry on paraffin-embedded tissues because it preserves overall tissue architecture by forming cross-links between proteins, while remaining compatible with antigen retrieval methods that unmask epitopes for antibody binding. This balance—stable morphology plus workable antigenicity after retrieval—makes it the go-to choice in most labs.

Bouin's solution, while excellent for morphology in some contexts, often masks many antigenic sites due to its strong fixation chemistry and contains picric acid, which can interfere with staining and tissue handling in IHC. Acetone and ethanol fix tissues mainly by precipitating proteins and can cause tissue shrinkage or morphological distortion; they’re more commonly used for cytology, frozen sections, or special antigen requirements rather than as the universal primary fixative for FFPE IHC.