Von Behring demonstrated in 1913 that preparations of diphtheria toxin neutralized with antitoxin, similar to those which had previously been employed in immunizing horses, may safely be used for immunization of human beings. The effectiveness of active immunization against diphtheria in reducing both the incidence of the disease and the number of deaths caused by it was established by Park and his associates in the New York City Health Department. Thus encouraged, immunization against diphtheria has been practiced extensively, especially in America.
Until recently, the immunization has been accomplished by injection of standardized preparations of diphtheria toxin neutralized by antitoxin. The mixtures used by different workers varied in the proportion of toxin to antitoxin. Some were overneutralized, some just neutral and others were underneutralized. The total amount of toxin in an immunizing dose has also varied; 0.1 L + doses of underneutralized toxin have been substituted for the 3 L + doses
DICK GF, DICK GH. IMMUNIZATION AGAINST DIPHTHERIACOMPARATIVE VALUE OF TOXOID AND TOXINANTITOXIN MIXTURES. JAMA. 1929;92(23):1901–1903. doi:10.1001/jama.1929.02700490001001