ACTION OF MICRO-ORGANISMS 205 



become killed they form small round white corpuscles called 

 pus corpuscles. Suppuration is the result of the killing of 

 the white corpuscles, probably brought about by lack of 

 oxygen when the corpuscles are thickly massed in one place. 



The power of engulfing the bacteria depends on something 

 else than the mere presence of bacteria and white corpuscles, 

 because if the white corpuscles are thoroughly washed so as to 

 remove all traces of blood serum, they will not engulf bacteria. 

 The addition of a little blood serum to the mixture of bacteria 

 and white blood corpuscles causes the corpuscles to engulf the 

 bacteria. Wright and Douglas have ascribed this to the 

 presence of something in the serum which renders the bacteria 

 palatable to the corpuscles ; this they term an opsonin.* 



Chemical protection can be best illustrated by the case of 

 diphtheria. The horse is relatively resistant to diphtheria, so 

 that if a small dose of diphtheria toxin is injected into it there 

 is practically no effect on the horse except a slight rise in 

 temperature. It is found that afterwards the horse can 

 withstand a larger dose of toxin. Increasing doses of diphtheria 

 toxin are given until the animal is said to be immunised. By 

 mixing some of the blood serum from such an immunised 

 animal with diphtheria toxin it is found that a certain amount 

 of toxin is neutralised. This is shown by the absence of 

 symptoms when a small amount of the mixture is injected into 

 a susceptible animal. An antitoxin is said to be produced in 

 the immunised animal. The relation between toxin and anti- 

 toxin is probably some sort of chemical union or adsorption, f 



Not only can antitoxins be produced by the injection of 

 toxins but there are a whole series of substances, of a pro- 

 tective nature, formed by injection of various substances. 

 The stimulating action of serum on the phagocytosis of 

 bacteria by white blood corpuscles, can be increased by 

 inoculation of bacteria, even if the bacteria have been killed 

 beforehand. That is the opsonins are increased in amount. 



Injection of bacteria also leads to the production of sub- 

 stances which cause even motile bacteria to fall together in 

 clumps, thus interfering with their activity. We say that an 

 agglutinin has been produced. Injection of foreign proteins 

 leads to the production of some substance that precipitates 

 these proteins (precipitins) and injection of foreign red blood 

 corpuscles leads to the production of something which causes 

 haemolysis or destruction of these corpuscles (haemolysins) . 



* A. E.Wright and S. R. Douglas, Proc. Roy. Soc., 1903, vol. 72, p. 357. 

 t Sv. Arrhenius, Ergeb. d. Physiol., 1908, vol. 7, p. 480. 



