http://www.mansfield.ohio-state.edu/~sabedon/biol2075.htm

Once an antigen is bound by an antibody, any of four actions may occur:
1-the antigen may no longer function properly (see virus inactivation by antibody)
2-the antigen and/or attached microorganism may clump (a process called agglutination) or even precipitate out of solution
3-complement may be activated
4-antibody-dependent cell-mediated cytotoxicity may occur if the antigen is found on the surface of one of the body's own cells