INTRODUCTION 9 



geneous solution of a substance, e.g. a toxin, combine in 

 most cases with the injected body to form more innocuous 

 compounds. If these compounds are soluble in the mix- 

 ture, we call the antibodies antitoxins (at least if the injected 

 body is poisonous). If, on the contrary, the compound is 

 insoluble, the antibody is called a precipitin. Such pre- 

 cipitins are produced after the injection of different 

 albuminoid substances. 



If the injection is a suspension of cells, the antibody 

 formed sometimes dissolves these cells ; in this case it is 

 called a lysin. Thus after the injection of bacteria there 

 are formed bacteriolysins, which under proper conditions 

 dissolve the bacteria of the injected variety. After the 

 injection of erythrocytes so-called haemolysins are formed, 

 which produce haemolysis, i.e. cause the red colouring 

 matter (the haemoglobin) of the injected erythrocytes to 

 leave them and to diffuse into the surrounding liquid. 



In other cases, other antibodies are formed, often simul- 

 taneously with lysins, so-called agglutinins, which produce 

 an agglutination of the injected cells. In this case the 

 presence of salts plays an important role. 



The normal serum often contains a certain quantity of anti- 

 bodies. Thus, for instance, in the normal serum of horses re 

 markable quantities of diphtheria-antitoxin and of antirennet 

 are often found. This peculiarity is so frequently observed, 

 that Ehrlich supposes that all possible antibodies exist in 

 normal sera, though in most cases the amount is not suffi- 

 cient to be shown experimentally. In the blood of many 

 animals we find two organic substances, cholesterin and 

 lecithin, which react with many of the injected poisons and 

 therefore play an important r61e as antitoxins or even as 



