372 DEFENCE 



are those which remain locahsed in one part of the body 

 and secrete a great quantity of powerful toxin which 

 circulates in the blood. It is through the generaUsed effect 

 of their toxins that they kill. Such are the bacilh of 

 diphtheria and tetanus. In the other class are those which 

 have a greater capacity for reproduction and become dis- 

 seminated through the body. Their capacity to form dif- 

 fusible toxin is much smaller than in members of the first 

 class. The toxins are therefore not found in any quantity 

 away from the bacteria themselves. The greater number of 

 bacteria belong to this class : Bacillus Typhosus, Bacillus 

 Coh, Pneumococcus and many others. It is sometimes 

 stated that the first class form ectotoxins, the second endo- 

 toxins. Probably both classes produce ectotoxins, the 

 difference being one of degree of diffusion of the toxin. 



In order that we may understand how the body protects 

 itself against the harmful effects of toxins, let us first con- 

 sider how it behaves towards poisonous substances of simple 

 and known constitution. In the chapters on Metabohsm 

 we have come across several instances where the absorption 

 or injection of a substance leads to the excretion of that 

 substance by the kidney in a combined form which is not 

 toxic. When phenol, scatol or indol enter the blood-stream 

 they are excreted as the non-toxic sulphates. Organic 

 acids, such as aceto-acetic acid are excreted as the ammo- 

 nium salt. Such a mechanism is known as Protective 

 Synthesis. The most instructive example for our present 

 purpose is the excretion of benzoic acid combined with 

 glycine to form hippuric acid. Benzoic acid is toxic be- 

 cause it has an affinity for some essential chemical grouping 

 of the hving cell. By combination with glycine this 

 affinity can be satisfied. Now when a certain dose of 

 benzoic acid is administered, glycine is produced far in 

 excess of the amount required to combine with the benzoic 

 acid. Here, then, the body produces a protective sub- 

 stance and produces this substance in excess. Let us 

 now compare wdth this simple instance the behaviour of 



