Conformational changes and complement activation induced upon antigen binding to antibodies

The interaction between antibodies specific for the loop region of lysozyme and this monovalent antigenic determinant was compared to their interaction with either the dimeric derivative of the loop (bis-loop) or the polyvalent antigen loop-A--L. This comparison was based on complement fixation capacity, and on spectroscopic changes in the circular polarized fluorescence (CPL) of the antibodies. The binding of the loop to its antibodies causes spectroscopic changes, assigned to conformational changes in the Fc region, which are not accompanied by complement activation. However, the binding of the bisloop led to distinctly different CPL changes and concomitantly to complement fixation comparable to that induced upon binding of the loop-A--L. Complement fixation and the CPL changes can be induced in the intact antibodies only; reduction of the interchain disulfide bridges, or cleavage by papain or pepsin digestion abolishes both activities.