The opsonic properties of immune γG-globulins isolated from patients with chronic septicemic conditions, principally subacute bacterial endocarditis were studied. Opsonic capacity as well as complement-fixing properties of γ-globulins appeared to be closely associated with integrity of Fc structures. Progressive pepsin digestion of immune γG-globulins, as monitored by successive loss of Gm(a) and Gm(b) antigens, abolished opsonic activity. Colostral γA, containing agglutinating antibacterial antibodies but no demonstrable complement-fixing activity, was devoid of opsonic capacity. Reduction of γ-globulin opsonins with 0.01 or 0.1 M mercaptoethanol progressively abolished opsonic activity in parallel with loss of ability of treated γ-globulins to fix complement with bacteria. Treatment of γ-globulin opsonins with 0.01 M sodium metaperiodate also produced complete loss of opsonic capacity in parallel with loss of Gm(b) Fc antigens. These findings, together with antiopsonic effects demonstrable with anti-γ-globulin factors showing primary reactivity with Fc structures, indicate that the opsonic property of immune γ-globulins requires the participation of structures integral to the Fc region of γ-globulin.

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