diately appear. There has been formed a new substance that does not occur 

 in normal blood serum. This new precipitating substance, or precipitin, 

 will precipitate only white-of-egg. If a different kind of protein is used, the 

 precipitin formed will act on that only. That is, the precipitin is specific. 

 We do not know how the protoplasm of an animal produces precipitin, 

 but we can use what we do know about precipitins in several ways: (1) We 

 can tell whether a bloodstain was produced by human blood, let us say, 

 or by the blood of some other species. (2) We can tell, by the precipitin 

 test, what kinds of meat there are in a sausage or hash, when all other 

 tests fail. 



A. With a small 

 syringe, blood 

 is removed 

 from the 

 suspected 

 patient 



cind left 

 to clot 



C. Serum 

 from the 

 clotted 

 blood 



B. In the meantime a 

 clean growth of typhoid 

 bacilli is 

 made 

 ready 



In this growth bacteria move 

 about freely and singly 



^i^ 



is mixed 



with a 



'1 



-T) 



Vieo 



sterile salt solution in different proportions 



D. To a drop of each senim-salt mixture 



there is added a drop of typhoid culture 



If bacilli stick together even in dilute serum, 



the patient probably has typhoid fever; 

 If the bacilli remain apart even in concentrated 

 serum, he is probably not infected with typhoid 



WIDAL'S AGGLUTINATION TEST FOR TYPHOID 



241 



