Agglutination of sheep erythrocyte-rabbit antibody complexes treated with sheep serum by a rabbit antiserum specific for the third component (C3) of sheep complement.

Sheep E-rabbit A incubated with sheep serum for up to 20 min at 39 degrees C and washed were agglutinated by a rabbit antiserum (anti-inulin-foetal lamb serum) that was specific for sheep C3. If incubated for longer than 20 min at 39 degrees C, there was less sensitisation. Such a complex was not readily generated at 17 degrees C whereas there was no reaction if incubated at 2 degrees C. Once the sheep E-rabbit A-sheep C complex had been generated at 39 degrees C and washed free of sheep serum, it could still be agglutinated after overnight storage at 4 degrees C. Addition of chelators of Ca2+ and/or Mg2+ or antrypol or cobra venom factor (a specific inhibitor of C3) prevented sensitisation of sheep E-rabbit A by sheep serum for agglutination. These results show that sheep serum C3 was taken up by sheep E-rabbit A and its presence on this indicator demonstrated by agglutination using a rabbit antiserum specific for sheep C3.