The protective capacity of immune sera in experimental mouse salmonellosis is mainly due to IgM antibodies.

Passive immunization was used to protect mice against a general infection caused by Salmonella typhimurium and our purpose was to compare the protective capacity of different immunoglobulin isotypes (classes and subclasses). Three antisera were studied, one pool of mouse serum against the envelope of rough bacteria, and two rabbit sera against smooth bacteria. Three different methods were used to separate isotypes. The consistent finding was that only IgM antibodies protected efficiently. A unit of IgG antibodies had an effect that was 1/50th of the IgM effect or less. This effect could have been due to a contamination by IgM. IgA appears to be non-protective like IgG. In two of the antisera a considerable proportion of protective antibodies were against a defined antigenic determinant (anti-0-4,5 or anti-0-9). IgG antibodies of these sera measured by the solid phase assay were also predominantly anti-0-4,5 or anti-0-9, respectively. This argues that the failure of IgG antibodies to protect cannot be explained by assuming that unlike IgM antibodies they are directed against "non-protective" determinants. We conclude that the observed difference between the protective capacities of IgM and IgG antibodies is due to C-region differences between the mu- and gamma-chains.