Three sets of experiments were carried out on undergraduate medical students at Oxford, Cambridge and Liverpool during the years 1941–4 in order to supplement the information obtained previously (Downie et al. 1941) on the comparative antitoxin response of those given active immunization alone (Group A) and those given combined active and passive immunization (Group A+P). A summary of each of the experiments has already been given in the text, so that it is unnecessary here to do more than recapitulate briefly the main results.

1. The first experiment showed that in Group A the antitoxin response was not appreciably greater in students receiving doses of 0·3 and 0·3–0·5 ml. of A.P.T. at 4 weeks’ interval than in those receiving doses of only 0·1 and 0·3 ml. In Group A + P no difference was noticed in the antitoxin content of the serum 6–8 weeks after the second injection of A.P.T., but 10–12 weeks after the second injection there was a difference in favour of the students receiving the larger doses of A.P.T., though it was below the conventional level of statistical significance.

2. The second experiment showed that when the doses of A.P.T. were spaced by 2 instead of by 4 weeks the antitoxin response was much less in both the A and the A + P groups, though the difference was less in the latter group, particularly when the measurements were made 10–12 weeks after the second inoculation of A.P.T.

3. The third experiment showed that a dose of 5000 units of diphtheria antiserum given at the time of the first injection of A.P.T. inhibited antitoxin production to a greater extent than a dose of 400–500 units, though the difference was much less when the measurements were made at 12 weeks after the second inoculation than at 4 weeks.

A compilation of the results obtained during 1940–2 in groups of students receiving active and those receiving active plus passive immunization shows that the antitoxin production in the first group (Group A) was much higher than in the second group (Group A + P) 6–8 weeks after the second injection of A.P.T., but that 10–12 weeks after the second inoculation the difference, though still significant, was considerably less. The final Schick-test results at 10–12 weeks gave a Schick-conversion rate of 98.0% in Group A and of 90.9% in Group A + P.

A review of the results obtained during the years 1939–44 on about 450 students at Oxford, Sheffield and Liverpool leads to the conclusion that the effect of giving diphtheria antiserum at the time of the first injection of A.P.T. is to cause a delay and some degree of inhibition in the antitoxin response of the subject. The larger the amount of antiserum given, the greater is this effect. With a dose of 500 units, though the delay in antitoxin formation is very obvious 4 weeks after the second injection of A.P.T., the final degree of immunity attained, as judged by the antitoxin concentration of the blood serum and by the Schick-conversion rate, is not greatly inferior to that resulting from active immunization alone; and even with a dose of 5000 units, the Schick-conversion rate reaches 81 % 12 weeks after the second injection of A.P.T. It is clear, therefore, that the antiserum, even when given in a dose as large as 5000 units, does not neutralize more than a small part of the antigenic activity of the first dose of A.P.T. Its main effect is apparently to diminish the rate of sensitization of the tissues, so that when a second dose of A.P.T. is given 4 weeks later, the rise in the antitoxin content of the blood serum is considerably delayed. Our experiments suggest that by increasing the size of the first dose of A.P.T., some of this delay may be avoided.

The partial neutralization of the first dose of A.P.T. will result in a decrease in the total antigenic stimulus and a delay in the time at which ït comes into operation. It is suggested that, provided the tissues have not been previously sensitized to diphtheria toxin, the result may be that the two doses will act virtually as a single dose. Such an explanation, however, must remain unproven till further observations have been made (see p. 35).

The practical value of combined active and passive immunization, especially when joined with temporary segregation of healthy carriers, in combating outbreaks of diphtheria in schools and other institutions for children has been clearly shown by Fulton, Taylor, Wells & Wilson (1941). Our present experiments lead us to suggest that, when applying the method in practice, it would be wise to give an initial dose of 0·5 ml. of A.P.T., together with 500 units of diphtheria antiserum injected at a different site, followed 6 weeks later by a second dose of 0.5 ml. A.P.T. It is probable that children treated in this way will develop approximately the same ultimate degree of immunity as those actively immunized with doses of 0·3 and 0·5 ml. of A.P.T. at 4 weeks’ interval.

We should like to express our thanks to Prof. A. D. Gardner and Prof. H. R. Dean for permitting observations to be made on the students in the pathology classes at Oxford and Cambridge; and to the students themselves at Oxford, Cambridge and Liverpool, for their ready co-operation in the inquiry.