Protein-Binding Properties of Immunoglobulins

Complement is a group of 11 serum proteins which act on cells through a sequence of events initiated by fixation to antigen-antibody complexes. The classical complement pathway is started by the fixation of the first three complement components as a complex. These three components (Clq, Clr, and Cls) are referred to collectively as Cl and are associated with each other noncovalently in the presnce of Ca ion. Clq represents the recognition unit of the aggregate which is bound by the antigen-antibody complex and in turn causes the conversion of Cls (conformational change ) into a proteolytic enzyme. Cls then activates the next component (C4) of the sequence. For a full review of the complement system, the reader is directed to the chapter by Stroud in this volume. 

Several laboratories reported that the Fc fragment of IgG contains the site involved in the fixation of complement (Amiraian and Leikhim, 

Ellerson et al. (1972) replaced plasmin with trypsin in the acid exposure proteolysis treatment of human IgGl Fc fragment and isolated the Ch2 domain. They were then able to demonstrate CF with this fragment. 

As previously discussed, amidination and carbamylation cause the loss of CF activity (Cohen and Becker, 1968a). In addition, diamino compounds can inhibit the Cl-antibody interaction (Sledge and Bing, 1973). Yasmeen et al. (1976) have, in fact, shown that diaminobutane reacts with Cl and had a dose curve parallel to that of Fc fragment but 

IgG is the lymphokine, IBF (immunoglobulin-binding factor). This has been reported to be a T-cell product which binds the Fc region of IgG close to the complement-binding site (Fridman and Golstein, 1974). Its biologiced significance is unknown. 

---