The effect of limited fragmentation of porcine 19 S IgM on agglutination and complement fixation. Evidence that at least two intact subunits are required for complement fixation by IgM antibody.antigen complexes.

Limited fragmentation of pig IgM anti-Salmonella with pepsin gave a 12 S product which agglutinated Salmonella almost as efficiently as the original IgM but the complement fixing ability of the complexes was substantially lowered. The 12 S product was shown to consist of molecular entities having intact Fc regions with an average of 3 or 4 intact Fab arms still attached. Our results appeared to show that removal of Fab arms by pepsin was a random process and on this basis a mathematical model was set up. From this model it was possible to calculate the frequencies of molecular entities which would give rise to the observed average of 3 or 4 Fab arms. The most likely explanation of our results is that complexes need to be formed by molecules having at least two intact subunits before efficient complement fixation takes place.