Influence of anti-fragment A of diphtheria toxin on passive hemagglutination with antitoxin.

Hemagglutination (HA) and toxin neutralization (TN) tests were used to titrate human and guinea-pig serum specimens taken at various stages of immunization for diphtheria antitoxin. The ratio of HA to TN titers varied significantly depending on the immune status. The ratios and the range of their variations became larger and high values exceeding five were often obtained after repeated booster immunization. Such a high value was proved to be related to the antibody against fragment A (FrA) of diphtheria toxin, since the ratio reduced significantly when anti-FrA was absorbed. Anti-FrA was not produced in children after the basic immunization with diphtheria toxoid and was detected in only one-third of vaccines after the booster injection given after 1 year. It was produced abundantly, however, when booster immunization was repeated. The pattern of production of anti-FrA in guinea pigs was similar to that in humans, when immunized with diphtheria toxoid with adjuvant. No anti-FrA was produced even when the animals were immunized repeatedly with plain diphtheria toxoid.