Complement Utilization by Guinea Pig γ1 and γ2 Immunoglobulins through the C3 Activator System

Abstract

The recent demonstration of the C3 activator system in human serum (1) suggested a possible mechanism for the utilization of late complement components by guinea pig γ1 immunoglobulins which, as was previously shown, bypass interaction with C1, C4 and C2 (2–4). Antibodies with the γ2 mobility activate the C sequence at C1 and in addition may follow the same pathway of late component utilization described for the γ1 variety. Both of the guinea pig immunoglobulins possess a site on the F(ab′)₂ fragments for activation of the C sequence at the C3 stage. The site for initiation of the classical C sequence, possessed only by the γ2 immunoglobulins, resides on the Fc fragment since it is destroyed by treatment of the molecule with pepsin. The C3 activator system has also been shown to furnish an alternate pathway for the activation of the late complement components (C3–C9) (1).