1. Schick tests and antitoxin titrations were performed on 143 subjects, divided into the following groups:

    • Subjects with no demonstrable diphtheria antitoxin in their blood (<0.0005 A.U./ml serum; table 2);

    • Subjects with diphtheria antitoxin in their blood due to active immunization (figure 2, table 3);

    • Subjects with diphtheria antitoxin in their blood due to passive immunization with homologous antidiphtheric serum (figure 3, table 4).

  2. The results obtained permit the following conclusions as to the interpretation of the Schick test: The comparatively wide individual variations in the ratio between the size of the Schick reaction and the antitoxin titer of the blood are only partially explained by potential immunity. Non-specific individual differences of the skin play a role, which should not be neglected for the specific tissue factors. The possible implication of these findings for other skin tests is briefly discussed.

  3. On the basis of the results presented in this paper the following interpretation is given to Schick reactions of varying types.

A completely negative reaction is evidence of a previous antigenic influence and an antitoxin-titer usually above 0.002 A.U./ml.

A positive reaction, with a diameter of redness below 15 mm, is evidence of an antitoxin-titer always below 0.02 A.U./ml.

A strong positive reaction with a diameter of redness at or above 15 mm is evidence of an antitoxin-titer always below 0.005 A.U./ml.

1

Parts I–III of this series were published in Acta Paediatr., vol. 35, 1948.

2

This investigation was supported by a grant from the Medical Research Council of Sweden. The authors wish to express their thanks to Dr. J. Ström, head of the Hospital for Infectious Diseases in Stockholm, for facilitating our search for suitable high-titer blood donors. Our thanks are due also to all those pupil nurses and medical students who volunteered in our experiments.

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