So 



THE PRINCIPLES OF IMMUNOLOGY 



in some detail, but it must be pointed out at once that the specificity 

 is not absolute. It was soon found that blood-cells and later 'other 

 body cells could be agglutinated by specific sera. It was also 

 found that agglutinins of various kinds exist normally in certain 

 sera, these being called normal agglutinins as opposed to the arti- 

 ficially produced or immune agglutinins. It was found that the 

 agglutinins resist heat of 56 C, a degree sufficient to destroy com- 

 plement, and that after being rendered inactive by heat cannot be 

 reactivated by fresh normal serum. It was soon observed that in 

 the course of infectious disease due to a specific organism agglu- 

 tinins^are likely to develop, and this led to the discovery in Widal's 



FIG. 4. Method of obtaining blood from the posterior auricular vein of the rabbit's ear. The 



vein has been incised by means of a small hypodermic needle. The same position of the animal 



serves for intravenous injections which are given into the posterior auricular vein. 



clinic in Paris, a few months after Gruber and Durham's publica- 

 tion, of the now widely used Widal reaction for typhoid fever. Con- 

 versely with a serum of known type, the antigenic bacteria may be 

 identified. The demonstration of agglutination may be by the 

 microscopic method or by the macroscopic method. In our pre- 

 sentation of the subject it is considered desirable to illustrate the 

 points by actual experiment, and for this reason we proceed to take 

 up the method of producing immune agglutinins in the laboratory 

 and subsequently present the factors which qualify and modify 

 the process of agglutination. 



Production of Immune Agglutinins. Injections for producing 

 agglutinins may be subcutaneous intraperitoneal, intravenous, or a 



