IMMUNISATION 197 



antibodies is a specialised branch of the subject outside the 

 scope of this book, but one obvious method of combating 

 infection is to assist the production of the antibody against 

 the infecting agent. This can be done in various ways, 

 which have varying effectiveness against different organisms. 

 In cases where the infection is already established and toxaemia 

 present, it is sometimes possible to inject the specific anti- 

 toxin itself. Antitoxin is made by injecting sub -lethal doses 

 of toxin into a large animal such as a horse, removing the 

 plasma after antitoxin formation has occurred, and using some 

 preparation of this plasma as a source of antitoxin. It is 

 more satisfactory to produce antitoxin in the blood of the 

 host itself and, if possible, to produce this prior to infection 

 so that accidental contamination with the pathogen will not 

 advance into virulent infection. Such " active immunisation " 

 €an often be accomplished by the injection of some harmless 

 preparation of the toxin or organism and so stimulating the 

 antibody response that the antibody concentration in the 

 blood-stream will remain effective for some considerable 

 time after immunisation. To stimulate such a response 

 three main types of preparation are used: (1) a " vaccine " 

 consisting of a suspension of organisms which have been 

 killed either by heat or by chemical treatment and are conse- 

 quently non-viable; (2) preparations of the toxin itself, 

 injected in minute doses at first and then in increasing doses 

 at intervals until a sufficient antitoxin response has been 

 built up; (3) toxoid preparations — if the toxins of some 

 organisms such as CI. ivelchii, CI. tetatium, Corynebacterium 

 diphtheriaef etc., are treated with a weak solution of formalde- 

 hyde, their chemical structure is altered in some way which 

 results in the destruction of their toxic nature but not of their 

 ability to stimulate antitoxin formation; consequently, in 

 these cases, it is possible to inject a^relatively large amount of 

 " toxoid " to stimulate a correspondingly large antibody 

 response without any toxic effect on the host. All of these 

 methods are used to combat specific infections and each has 

 its advantages in certain conditions, but all are dependent 



CHEM. A. B. 13* 



