Sera of newborns show a much lower content of antibodies against E. coli, serotypes 055 and 0111, than the corresponding maternal sera.The bactericidal test yielded a higher percentage of positive results than the hemagglutination test with sensitized erythrocytes, and the titration of inactivated sera in the presence of a constant amount of guinea pig complement gave higher titers than the titration of active human sera without added complement.The hemagglutination titer for erythrocytes sensitized with extracts of pathogenic E. coli or Shigellae rises from the third month until the peak of the average titer for all enteric pathogens tested is reached in the fourth year of life.The highest percentage of positive reactors is reached for most of the strains at the age of 15 years, indicating that after the age of 5 years further contacts with these pathogenic microorganisms still take place.
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