PASSIVE IMMUNITY 465 



increased doses of a toxin of a particular microbe, may protect 

 an animal B against a certain amount of the same toxin when 

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

 would 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', (6) it may 

 produce an increased susceptibility to ingestion by phagocytes 

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

 agglutinative action.) 



These two kinds of anti-sera antitoxic and antibacterial 

 exert their effect when injected along with the toxin or organism 

 respectively or some time previously ; as would be expected, 

 they have less effect when injected some time afterwards, though 

 even then they may have a certain degree of curative or palliative 

 power. The two properties, antitoxic and antibacterial, are essen- 

 tially different in kind, the former leading to a neutralisation of 

 the toxin, the latter to some alteration in the vital activity of 

 the bacterium ; in other words, the point of attack in the case 

 of the two sera is different. A serum may, however, possess 

 both properties in varying degree. The fundamental fact in 

 passive immunity, viz. 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. It will accordingly . 

 be convenient to speak of anti-substances in general. 



The development of anti-substances, 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 

 those which have not this property. Amongst the former are 

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