The stabilization of the C3b molecule in immune deposits by formalin

Formalin-fixed tissue may be used as substrate for immunohistochemistry after enzymatic treatment with proteases. However, whether the antigenicity of C3b is restored is controversial owing to the protease sensitivity of the native molecule. The antigenicity and protease sensitivity of formalin-fixed and non-fixed C3b were tested by using C3b-opsonized yeast particles and the immunofluorescence technique. The results indicate that formalin, probably due to cross-linking, stabilizes C3b molecules and renders them less susceptible to protease treatment. Failures to detect C3b in immune deposits in fixed tissues may be due to factors other than protease treatment.