Hemagglutinating 7S subunits of 19S cold agglutinins.

Two highly purified IgM cold agglutinins have been mildly reduced yielding 7S subunits, with interchain covalent bonds intact. These subunits retained most of the cold-agglutinin activity as well as the specificity of the parent antibodies. However, as might be anticipated from theories of the importance of antibody size and number of subunits for complement binding, the IgM subunits were only very weakly lytic compared with the intact cold agglutinins. The findings are consistent with the presence of ten antibody-combining sites on the IgM molecule.