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The association of complement activation at a low temperature with hepatitis C virus infection in comparison with cryoglobulin

Complement activation at a low temperature in vitro and cryoglobulinaemia are associated with hepatitis C virus (HCV) infection. The frequency of HCV antibody positivity determined in serum specimens that showed the cold‐dependent activation of complement was 100%, whereas it was 48% among sera with cryoglobulin. On the other hand, the frequency of cold activation among HCV‐infected sera was 41%, and that of cryoglobulin 48%. Cold activation was not found in any HCV− sera studied, whereas cryoglobulin was found at a frequency of 14% in HCV− sera. Cold activation was also absent among hepatitis B virus (HBV) S antigen or antibody‐positive sera, except a few that were both HBV+ and HCV+. Rheumatoid factor was also frequently detected in sera with cold activation or cryoglobulin. Cold activation and cryoglobulin may be generated by common mechanisms in which a low avidity, low temperature‐preferring antibody may function. In sera with cold activation, fine particles of immune complexes, which do not form precipitates, may activate the complement system. HCV is a unique virus that coexists with antibody in the serum, therefore the avidity of the antibody for the virus antigen may be low, and occasionally react only at a low temperature. This may be why the in vitro phenomenon related to immune complexes occurs specifically in HCV‐infected sera.