i8 9 6. SERUM THERAPEUTICS. 107 



The reasons for accepting some such theory are shortly as follows. 

 It has been known for some time that the freshly drawn blood of 

 certain animals possesses germicidal power, and Buchner showed that 

 such power resided in the serum. It is destroyed by heat. The body 

 to which it is due appears to be a proteid, and in some cases, at least, 

 a globulin : Buchner and also Hankin have isolated a germicidal 

 globulin from the spleen and serum of rats. The germicidal power 

 varies widely according to the animal from which the serum is 

 derived, and is much more marked upon some species of micro- 

 organism than upon others. Speaking broadly, the serum from 

 an animal which is naturally immune against a given pathogenic 

 organism possesses the greatest germicidal power for that organism, 

 but there are exceptions to this rule. This germicidal power of 

 animal blood is not the only one concerned. The action of pathogenic 

 organisms in producing disease is mainly a chemical one. In many 

 instances, such as tetanus and diphtheria, the bacilli remain localised 

 at the seat of infection, and the severe constitutional effects of the 

 disease are produced by the absorption of the virulent chemical poisons, 

 or " toxins," to which they give rise. The existence of such toxins is 

 not hypothetical : they have been recovered from the bodies of patients 

 who have died, and they are readily produced in artificial cultures of 

 the bacilli in question, from which they can be separated by filtration 

 through unglazed porcelain. With the sterile filtrate, containing the 

 toxins in solution, the phenomena of the disease can be in large 

 measure reproduced in animals. The toxins have been found to be 

 proteid bodies. Now it has been shown that, under certain circum- 

 stances, a substance is present in animal blood which can neutralise 

 the toxins produced by pathogenic organisms even when it has not a 

 direct germicidal action upon the organisms themselves. This, then, 

 is a property of a different kind, but of equal importance : to this sub- 

 stance the term " antitoxin " should in strictness be applied. It has 

 been found that serum from animals which have been artificially im- 

 munised against certain diseases by inoculating them with an attenuated 

 virus is able to neutralise the toxins produced by the bacilli of those 

 diseases, and also — a still more important step — that such serum 

 injected into susceptible animals confers upon them immunity 

 from the diseases in question. The first suggestions as to this im- 

 portant property were made in 1889 by Babes and Lepp in connection 

 with rabies, but the credit of first clearly demonstrating the principles 

 of serum therapeutics belongs to Behring and Kitasato, who in the 

 following year applied it to tetanus and subsequently to diphtheria. 

 They showed that serum from a rabbit which had been immunised 

 against tetanus protected mice against virulent tetanus cultures, and 

 further, that the toxins in a filtered culture of the tetanus bacillus were 

 neutralised by admixture with the blood of an immunised animal. 

 Similar facts have been shown for diphtheria, pneumonia, and some 

 other diseases. 1 2 



