ANTITOXIC SEBUM 561 



particular antigen which has led to its development, and we 

 shall discuss the evidence for this later. Furthermore, the anti- 

 substance has apparently a specific combining group which fits, as 

 it were, a group in the corresponding antigen, the two groups 

 having been compared to a lock and key. It is, however, to be 

 noted that this specificity is a chemical or physical one rather 

 than a biological one. An anti-serum, for example, developed 

 by the injection of bacterium A may also have some effect on 

 bacterium B, and thus appear not to be specific. It is known, 

 however, that the antigens in bacterium A are not all identical, 

 and that some of them may be present though in smaller pro- 

 portion in bacterium B ; thus the theory of combining specificity 

 is not invalidated. The number of different anti-substances, as 

 judged by their combining properties, would appear to be almost 

 unlimited, a fact which throws new light on the complexity of 

 the structure of living matter. When anti-substances are studied 

 as regards their action in vivo or in vitro on the substances with 

 which they combine, different degrees of complexity may be 

 recognised. In certain cases simple combination may occur 

 (antitoxins, antiferments), in other cases physical effects may be 

 associated with combination (agglutinins), and in a third group 

 of cases the anti-body may lead to the union of another body 

 normally present in serum, called complement or alexin. The 

 combination may or may not result in physical changes in the 

 antigen, the evidence of the latter occurrence being elicited by 

 the deviation method (p. 127). Anti-bodies of the third class 

 are known as immune-bodies or amboceptors (Ehrlich) or sensi- 

 tising substances, — substances sensibilisatrices of French writers. 



After this preliminary statement in explanation, we shall con- 

 sider the actual properties of the two classes of serum, and later 

 we shall resume the theoretical consideration. 



Antitoxic Serum. — In a previous chapter (p. 186) a distinction 

 has been drawn between exo- and endotoxins, and with regard 

 to these the general statement may be made that while antitoxins 

 are, as a rule, comparatively easily obtained in the case of the 

 former, the matter is quite otherwise in the case of the 

 latter. In fact some writers have gone so far as to say that 

 antitoxins to endotoxins cannot be obtained. Such an extreme 

 view is in our opinion unjustifiable in the light of the recent 

 work on antitoxins to the typhoid, cholera, and dysentery endo- 

 toxins (pp. 368, 470, 386). Nevertheless we have the im- 

 portant fact that in many cases by the injection of dead cultures 

 an active anti-bacterial serum can be obtained which has no 

 neutralising action on the endotoxins, and we must conclude 



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