522 IMMUNITY 



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 on the substances with which they com- 

 bine, they may be conveniently arranged in three classes 

 corresponding to Ehrlich's three classes of receptors (vide p. 549). 

 In the first place, the anti-substance may simply combine with 

 the substance without, so far as we know, producing any change 

 in it, and to this group the anti-toxins and anti-ferments belong. 

 In the second place, the anti-substance, in addition to combining, 

 may produce some recognisable physical alteration. In other 

 words, it possesses an active or zymotoxic group as well as 

 a combining group. The agglutinins may be mentioned as 

 examples of this group. In the third place, the anti-substance 

 after combination may lead to the combination of another body 

 normally present in serum called complement or alexine, and 

 this latter, which has a constitution very similar to that of a toxin, 

 may lead to physical change, for example, death or solution of a 

 cell. Anti-substances of this class are known as immune-bodies 

 or amboceptors (Ehrlich) or as sensitising substances substances 

 sensibilisatrices of French writers. Their essential feature is that 

 they lead to the combination or fixation of complement, which 

 may or not produce some recognisable change such as bacterio- 

 lysis, etc. If no such effect follows, however, the union of com- 

 plement may be demonstrated by the indirect or deviation 

 method (p. 130). 



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. 188) a distinction 

 has been drawn between extra- and intra-cellular toxins, 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 

 endotoxins (pp. 366, 456, 388). 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 

 either that a large proportion of the endotoxin does not lead to the 

 production of antitoxin or does so only with great slowness, the 

 latter alternative being on general grounds rather improbable. 



