Plasma cells are identical to the B-cell. They secrete loads of antibodies specific to the antigen. These are called monoclonal antibodies. They bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes.
An antibody has two bonding sites, so can bind to two pathogens at the same time. This means that pathogens become clumped together- this is called agglutination.