558 IMMUNITY 



injected along with the latter, or a short time before it. As 

 might be expected, it has less effect when injected some time 

 afterwards, but even then within certain limits it has a degree 

 of curative or palliative power. Seeing that the serum of animal 

 A appears to neutralise the toxin, the term antitoxic has been 

 applied to it. 



2. The serum of an animal A, highly immunised against a 

 bacterium by repeated and gradually increasing doses of the 

 organism, may protect an animal B against an infection by 

 the living organism when injected under conditions similar to the 

 above. This serum is therefore antimicrobic, or antibacterial, 

 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. 129) may be 

 shown by the deviation of complement method ; (6) 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, 

 glucosides, 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 



