INTRODUCTION 15 



tortus, Ricinus, etc. In general it may be said that 

 the diseases caused by pathogenic bacteria are 

 caused not so much by the bacteria themselves as 

 by the products secreted during their lifetime or set 

 free after their death. 



The human or animal body possesses means of 

 combating the action of these poisons. If they are 

 injected into the body, or even if the bacteria them- 

 selves are injected, the blood after some time con- 

 tains substances which neutralize the poisons or 

 act upon the bacteria. Such substances are called 

 antibodies, whereas the injected poisons or bacteria 

 are called antigens, i.e. bodies which cause the forma- 

 tion of antibodies. Later it has been found that 

 the injection into an animal of albuminous substances, 

 e.g. milk or egg-white, or serum or corpuscles from 

 the blood of non-related animals, which seem to be 

 comparatively harmless for the animal, causes the 

 production of antibodies. The antigens and the 

 antibodies are of extreme importance for the welfare 

 of man, and they have therefore been the object of 

 very extensive studies, mostly only of a qualitative, 

 but in recent time also of a quantitative character. 



The antibodies are divided into different groups, 

 according to their mode of action on the antigens, 

 as lysins (bacteriolysins, which cause the destruction 

 of the bacteria, or haemolysins, which let the haemo- 

 globin, the red colouring matter of the blood-cor- 

 puscles, go out into the surrounding fluid); precipitins, 

 which produce a precipitate with their antigens, agglu- 

 tinins, by the influence of which the antigens — in this 



