CHEMICAL AND PHYSICAL METHODS 4I 



the ordinary bacteriological methods, but by more subtle 

 methods called immunity reactions. These reactions are essen- 

 tially chemical, and cover all the processes by means of which 

 the body tries to fight against infectious disease. For example, 

 we know that if a solution containing microbes, let us say 

 pneumococci of type I, is injected several times into the veins 

 of an animal, the latter will, as a reaction of defence against 

 the foreign cells, manufacture new substances, called 'anti- 

 bodies', in his blood. The consequence of this fact is not only 

 that the animal is actively immunized against a subsequent 

 infection of living and virulent pneumococci but that the 

 serum (which is the liquid part of the blood) containing the 

 antibodies, confers a passive immunity, when it is injected into 

 the body of another anim^al susceptible to the same infection. 



What is more, when the serum of the animal immunized by 

 means of the pneumococci type I is mixed with the same 

 microbes in a test-tube, the micro-organisms act in a strange 

 way. They stick to each other in clusters, in masses. They 

 are what is called 'agglutinated'. These reactions are strictly 

 type-specific^ that is to say serum antipneumococcus type I only 

 agglutinates the pneumococci type I and has no action on the 

 organisms of another type. It is therefore possible, by em- 

 ploying specific immunized sera of each type of pneumococcus, 

 to classify an unknown culture by the nature of its reaction in 

 contact with one or other of the immunized sera. 



I insist on the specificity of the different types of pneumo- 

 cocci and on their immunological reaction thanks to which 

 they can be distinguished, for this is at the base of the work 

 which will now be described. 



The biological classification of pneurriococci was established 

 before anything was known of the chemical nature of the sub- 

 stances which determine the specificity of the type. Before 

 going any farther we will say a few words on the importance 

 of having a biological classification for the chemical and 

 epidemiological knowledge of the malady. This classification 

 has made it possible to appreciate the frequency of cases of 

 pneumonia due to each type of microbe, to recognize the 



