570 MICRO-ORGANISMS AND DISEASE [chap. 



alexines in glanders in an almost insusceptible animal, as 

 the tame mouse. Nor can the alexines which are present 

 in the tissues, and which act germicidally on saprophytic 

 bacteria, be the same as the alexines protective in insuscep- 

 tible animals against specific bacteria. Again, the alexines 

 protecting a naturally insusceptible animal against a specific 

 microbe cannot be the same as the substances protecting 

 against a second infection a susceptible animal which has 

 passed through one mild attack ; that is to say, the natural 

 immunity of an individual cannot be due to the same kind 

 of protective substance as the acquired immunity. 



Moreover, it has been shown that the inhibitory power 

 possessed by the blood (serum), though it can be greatly 

 increased and rendered specific against a particular species 

 of microbes by previous injections of a particular animal 

 with this microbe (artificial immunisation), may be and 

 sometimes is already naturally present in the normal 

 animal : Roux {Annales de Plnsi. Pasteur, September, 1894), 

 for instance, finds the blood-serum of a normal horse pos- 

 sessed of a certain high degree of resisting or inhibitory 

 power against diphtheria, Loffler {Centralbl. f. Bakteriologie, 

 February, 1896) finds the blood of a normal dog possessed 

 of inhibitory power against the typhoid bacillus, Cobbett 

 {Journal of Pathology and Bacteriology, January, 1896) finds 

 the blood-serum of a normal horse possessed of a certain 

 amount of inhibitory power against diphtheria toxin as also 

 against the living diphtheria bacilli. 



The essential and primary element in the resistance or 

 immunity of a tissue or of an animal against the growth and 

 multiplication of a microbe is the power of the tissue juices 

 (plasma, serum, or lymph) to injure or destroy the microbe 

 by virtue of its alexines, that then the so altered microbes 

 may be easily taken up by leucocytes (attracted there) and 



