Identification of the activator system for antibody to Toxoplasma as the classical complement pathway

In view of the many recent advances in our understanding of the composition and function of the complement system, it was decided to apply this newer knowledge to an investigation of the heat-labile activator required for the action of antibody to Toxoplasma gondii in the neutralization and dye tests. With use of antibody-coated toxoplasma trophozoites in a diluent of 0.2% gelatin in 0.85% NaCl and alkaline methylene blue as indicator, various component-deficient sera were added to activate the antibody on the protozoan membranes. It was determined that the classical complement system is required for antibody activity and that the properdin system plays no role in the reaction. Human sera genetically deficient in C5, C6, C7, and C8 were shown to be inactive as activators of antibody to Toxoplasma. The addition of specific missing components immediately restored full activity to the deficient sera.