“…-Littlewood et al (1954) urged that anyone who has received horse serum should be actively immunized not only against tetanus but also against diphtheria. Diphtheria antitoxin, like tetanus antitoxin, is produced in horses, and a second injection of this animal's protein may result in its very rapid elimination from the body, thus running the risk of greatly shortening the duration of passive immunity (Wright, 1958).…”