Activation of the third component of complement (C3) detected by a monoclonal anti-C3'g' neoantigen antibody in a one-step enzyme immunoassay.

A previously produced and characterized rat monoclonal antibody recognizing a neoantigen in the human C3'g' fragment was used as the capture antibody in an enzyme-linked immunosorbent assay. Detection was made using a polyclonal rabbit anti-human C3d and a peroxidase-linked anti-rabbit Ig antiserum. The activity in normal human EDTA plasma was found to be 3% of that in a zymosan-activated serum pool. Fractionation experiments revealed that most of the activity in normal plasma, in vivo activated plasma and in vitro activated serum eluted in one peak with a molecular weight corresponding to iC3b. A positive correlation (P less than 0.01) was found between the present assay and a previously established two-step C3d ELISA both with respect to normal plasma, individual patient samples and consecutively drawn samples following artificial in vivo activation. Complement activation assays based on specific antibodies to 'activation antigens' should be preferred whenever available since they enable direct, rapid and specific quantification of the actual fragment(s).