472 IMMUNITY. 



immunising the general health of the animal ought not to 

 suffer. If the process is pushed too rapidly the antitoxic 

 power of the serum may diminish instead of increasing, and 

 a condition of marasmus may set in and may even lead to 

 the death of the animal. (In immunisation of small animals 

 an indication of their general condition may be obtained by 

 weighing them from time to time.) 



Up till recently, the preparation from a horse of an antitoxic 

 serum of high value involved very prolonged treatment, usually lasting 

 for eight or ten months. Cartwright Wood has, however, devised a 

 method by which the period of immunisation is much shortened, and 

 which promises to give serum of very high antitoxic powers. In this 

 method he used two "toxines. " The one is the ordinary toxine 

 obtained from bouillon cultures as above described, which is believed 

 to contain the "ferments"; the other is obtained by growing the 

 diphtheria bacillus in a mixture of bouillon and 20 per cent of blood 

 serum (the latter is prevented from coagulating by having its lime salts 

 precipitated by oxalic or citric acid). Such a culture when filtered 

 contains the supposed ferments along with a large proportion of albu- 

 moses, produced by the action of the bacillus on the albumin. The 

 former are destroyed by exposure to 65 C. for an hour, and the fluid 

 is then known as "serum toxine" in contradistinction to the ordinary 

 "broth toxine." (It would be of importance to know whether or not 

 in this method the toxines are merely modified by the heating, i.e., 

 changed into toxoids, vide p. 162.) The serum toxine gives rise to 

 little local irritation but to marked febrile reaction. By its use the 

 early period of immunisation is much shortened, so that a horse can 

 tolerate a large dose of ordinary broth toxine in a shorter time than 

 was formerly possible ; and by combining its use with that of broth 

 toxine a serum of remarkably high antitoxic powers may be obtained 

 in a month or two. 



4. Estimating the Antitoxic Power of, or ' *" standardising" 

 the Serum. This is done by testing the effect of various 

 quantities of the serum of the immunised animal against a 

 certain amount of toxine. Various standards have been 

 used, of which the two chief are that of Behring and 

 Ehrlich and that of Roux. Behring adopted as the im- 

 munity unit the amount of antitoxic serum which will 

 neutralise completely 1 100 times the minimum lethal dose 



1 By this is meant not only that a fatal result does not follow, but also 

 that there is an absence of local swelling on the fourth day, and the animal 

 does not lose weight. 



