IMMUNITY 193 



Antigens and Antibodies. — The cells which play a 

 part in immunity are known: they are the phagocytes, the 

 micro- and macro-phages. The humoral properties correspond 

 to what are known as antibodies. 



The antibodies are the products (substances or properties) of 

 a reaction of the body towards a natural or artificial introduc- 

 tion into it of certain foreign substances, bacteria and their 

 poisons, vegetable poisons of other kinds, and various albumi- 

 noids all known by the name of antigens. The exact definition 

 of an antigen is its capacity of exciting in the injected (or 

 infected) body the production of an antibody. 



The discovery of the antibodies was so much more a splendid 

 biological acquisition in that its practical importance was at 

 least equal to its theoretical. The first antibodies studied were 

 the antitoxins of diphtheria and tetanus. The discovery of the 

 diphtheria toxin by Roux led to the discovery of its antitoxin 

 in the hands of Behring. The neutralisation in an ordinary 

 test-tube of a toxin by an antitoxin was one of the first and 

 most brilliant "/« OT^w " experiments in immunity. It might 

 certainly seem that this neutralisation could take place equally 

 simply in the living animal with no intervention of the cells, but 

 like a chemical combination. 



When an animal of species A is injected with the red blood 

 corpuscles of an animal of species B, the serum of the former 

 acquires the property of dissolving the globules of the other 

 species : it becomes haemolytic and the prepared animal is 

 said to have developed a hcemolysin. When the body is 

 vaccinated against the typhoid bacillus, the serum acquires 

 the property of agglutinating a homogeneous suspension of 

 typhoid bacilli : it is said to have produced an agglutinin. 

 The serum of an animal A which has been injected with 

 the blood or the serum of an animal B of a different species 

 forms a precipitate when to it there is added a little of the 

 serum B ; there is said to have been developed z, precipitin to B. 



Haemolysins, agglutinins, and precipitins are the antibodies 

 of which the blood corpuscles, the bacteria, and the serum- 

 proteins are the antigens. 



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