A new immunoassay for C3. Application of the cell-linked antigen radioactive antibody (CLARA) technique

The cell-linked antigen radioactive antibody (CLARA) method of immunoassay was applied to the C3 component of the complement system. C3 from whole complement was fixed on erythrocytes sensitized with IgM antibody. The IgG1 fractions of monospecific sheep anti-C3 were labelled with 125I and measurement of their specific binding to C3-bearing erythrocytes was used to quantitate the relative amount of fixed C3. Competititive inhibition of this specific binding was used to assay fluid phase C3. 0.3-0.6mug/ml of murine C3 were detected in the supernatant fluid of in vitro cultures of mouse spleen cells.