Enhancing Role of IgG in Lysis of Rabbit Erythrocytes by the Alternative Pathway of Human Complement

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Abstract
Adsorption of human serum with rabbit erythrocytes (E) or stromata at 0°C removes about 30% of its alternative pathway activity as measured by lysis of rabbit E in MgEGTA buffer. The activity is restored by adding back a low pH eluate from the stromata. Hypogammaglobulinemic sera have much lower levels of eluate activity than do normals. The activity of the eluate is totally abrogated by treating it with anti-IgG immunoabsorbant or Staphylococcal protein A. Reciprocal experiments comparing eluates from rabbit E stromata or zymosan revealed that eluate from zymosan had no restorative effect on the lysis of rabbit E by stromata-adsorbed serum and that the eluate from the stromata effected no increase in C3 consumption when zymosan-adsorbed serum was exposed to zymosan at 37°C. Conversely, each of the eluates enhanced alternative pathway activity when tested with their respective adsorbed sera. That this activity is not essential for alternative pathway activation was demonstrated by the ability of hypogammaglobulinemic sera to lyse rabbit E despite depletion of IgG to levels of less than 80 ng per ml as detected by radioimmunoassay. The results of these experiments suggest that specific IgG antibodies enhance, but are not essential for, lysis of rabbit E by the alternative pathway of human complement.