RESUMO
The immune response to Salmonella typhosa during hyperimmunization of dogs and rabbits by intravenous administration, at three different dose levels, three times a week for about seven months, was analysed. The pattern of immune response to both 'H' and 'O' antigen was determined, and antibody activity associated with IgM and IgG was clearly distinguished. Higher levels of both IgM and IgG activity were produced in rabbits than in dogs. The usual pattern of the early onset of IgM synthesis, with subsequent termination of active synthesis while IgG antibody was being formed, was observed in both groups. These results showed that with sufficient stimulation, hyperimmune anti-'O' sera may demonstrate a predominant IgG activity rather than the IgM activity considered to be characteristic of the response to bacterial lipopolysaccharides. Quantitative comparison of the amount of antibody produced to 'O' antigen by the quantitative precipitin method showed that the major differences between the dog and the rabbit remain, with a poorer formation of IgG antibody in the dog. The differences observed between the two species reflect the functioning of an antibody mediated feedback system at a lower threshold in the dog than in the rabbit.