424 HUMAN BIOLOGY 



observed by various methods, the most important of which 

 we can illustrate best in connection with tuberculosis. 



When an animal or a human being is tuberculous and we 

 reinfect the skin with tubercle bacilh or inject a small 

 amount of tubercuhn, the tuberculous animal will react 

 with a violent inflammation to a dose which would have 

 aroused little or no response in the normal. This means that 

 the body is on a hair trigger in regard to the specific response 

 which is set in motion by contact with this antigen, and 

 many of us believe that this specifically irritable condition 

 is a direct manifestation of the protective armament of the 

 cells. 



In most cases, however, the specific reaction aroused by 

 the antigen expresses itself not only by an increased cell 

 reaction, but by the appearance in the circulation of cell 

 products which we speak of as antibodies. Efforts have been 

 made to isolate these antibodies and although we have no 

 definite knowledge of either their exact chemical nature or 

 structure, we do know that they are associated with the 

 globulins of the blood plasma. We know them by their 

 activities rather than by their chemical and physical prop- 

 erties, in that we can easily demonstrate, by test-tube 

 experiment, that in the serum of an immunized animal the 

 particular bacteria with which the animal has been treated 

 undergo certain changes; and that by contact with the 

 bacteria the serum loses this particular property, that is, 

 the bacteria specifically absorb these substances. 



The changes which contact with an immune serum pro- 

 duces in bacteria as a consequence of this union are simple 

 and easily described: 



Bacteria which will remain finely suspended in a normal 

 serum will aggregate in clumps in a homologous immune 

 serum. 



Similarly, an extract of bacteria filtered clear and added to 

 a normal serum will leave the serum entirely unclouded, 

 whereas a similar extract of the same bacteria added to a 

 homologous immune serum will give rise to the formation 

 of a flocculent precipitate. 



In both of these cases an analysis of the mechanism has 

 shown that by union of the antibody and the bacterial 



