CHAPTER XXXVII 

 THE BLOOD AS A LIVING TISSUE 



The white corpuscles respond to the stimulation of foreign 

 substances in several distinct ways. Some of these are signifi- 

 cant in fighting diseases ; others have been used in different 

 ways ; still others we can neither understand nor utilize in 

 any practical way. 



223. Precipitins. When the white of an egg is taken into 

 the stomach of a backboned animal, it may be digested and 

 eventually absorbed into the blood or lymph, and from this to 

 the cells, where it acts as a food. But if some white of egg is 

 injected into the blood (of a rabbit, for example), it will have a 

 curious effect upon that blood. After a few such injections 

 the blood contains a substance that is not present in normal 

 rabbit blood. This new substance, which cannot be detected 

 by chemical methods, can be shown to be present if a drop or 

 two of the serum (see p. 181) of the treated rabbit is mixed 

 with a drop of water containing some white of egg. Imme- 

 diately there will be a visible precipitate. The substance that 

 is present in this serum, causing the precipitation of the egg 

 albumen, is called a precipitin, or precipitating substance. 



The formation of the precipitin by living cells is not at all under- 

 stood. An important fact about the precipitin is that it is always 

 specific. That is to say, a precipitin formed in an animal under the 

 influence of white of egg will precipitate only white of egg, but will 

 have no effect on other proteins ; a" precipitin produced under the 

 influence of a protein taken from a goat will precipitate only this 

 goat protein ; and so on. 



The fact of precipitin formation has been put into practical use 

 in several ways. 



