PASSIVE IMMUNITY 521 



i.e. preventive against invasion by a particular organism. (In 

 addition to the preventive or protective action in vivo, such a 

 serum may exert certain recognisable effects on the corresponding 

 organism in vitro. Thus (a) it may lead to the death or solution 

 of the organism bactericidal or lysogenic action ; when no such 

 effect follows, the presence of an immune-body (p. 128) may be 

 shown by the deviation of complement method ; (b) it may pro- 

 duce an increased susceptibility to ingestion by phagocytes 

 opsonic action ; (c) it may lead to the clumping of the organism 

 agglutinative action, or to precipitation with an extract of a 

 culture of the corresponding bacterium.) 



Anti-substances and their Specificity. The fundamental fact 

 in passive immunity, namely, that immunity can be transferred to 

 another animal, shows that the serum in question differs from 

 the serum of a normal animal in containing antagonistic sub- 

 stances to the toxin or bacterium as the case may be, these 

 being generally spoken of as anti-substances. The development 

 of these bodies, first observed in the case of the injection * of 

 toxins, is found to occur when a great many different substances 

 are introduced into the tissues of the living body. We can, in 

 fact, divide organic molecules into two classes those which give 

 rise to the production of anti-substances, and are thus known as 

 antigens, and those which have not this property. Amongst the 

 former are various toxins, ferments, molecules of tissue cells, 

 bacteria, red corpuscles, etc. They are all probably of proteid 

 nature, though their true constitution is not known, and none of 

 them have been obtained in a pure condition. Amongst the 

 latter may be placed the various poisons of known constitution, 

 glncosides, alkaloids, etc. We may also state at present that the 

 anti-substance forms a chemical or physical union with the 

 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 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. We have, however, 

 evidence to show that the antigens in bacterium A are not all 

 identical, and that some of them may be present though in smaller 

 proportion in bacterium B ; thus the theory of combining sjjeci- 

 t'n-ity is not invalidated. The number of different anti-substances, 

 as judged by their combining properties, would appear to be almost 



