456 IMMUNITY. 



its power to certain substances in it derived from the 

 spleen, lymphatic glands, thymus, and other tissues rich in 

 leucocytes. To these substances Buchner gave the name 

 of akxines. These substances are somewhat unstable 

 compounds, and are destroyed by the action of light, and 

 also by a temperature of 60 C. They can be precipitated 

 by alcohol and by ammonium sulphate, and correspond 

 in their general behaviour with enzymes or unorganised 

 ferments. Regarding the existence in the serum of 

 bactericidal substances which are very easily destroyed by 

 heat there can be no doubt, but their properties can only be 

 studied outside the body, and it must not be assumed that 

 the serum in such conditions has always the same property 

 as in the living body. In some cases, for example, the 

 bactericidal power of the serum in vitro has been found to 

 be greater than in a living animal. The bactericidal action, 

 moreover, is manifested towards some organisms and not 

 towards others, and this variation does correspond with 

 the immunity of the animal against these organisms. 



At present, therefore, the facts of natural immunity can- 

 not be fully explained. In some cases the insusceptibility 

 to toxic substances may explain the degrees of immunity 

 possessed by different animals, whilst in others immunity 

 may be due to special bactericidal powers possessed by them. 

 What these bactericidal powers really are cannot be explained 

 on any single theory. A vital activity of the tissues and 

 fluids is, no doubt, brought about by the presence of the 

 bacteria, and this cannot be fully imitated in experiments 

 outside the body. The facts given above with regard to 

 the action of antimicrobic serum, show how complicated a 

 matter the bactericidal process may be. Further, in natural 

 immunity a direct killing of the organisms by the fluids of 

 the serum is not necessary. It may be sufficient that their 

 growth is prevented, so that they ultimately die out or 

 are taken up by the phagocytes. 



