A GENERAL RESUME OF IMMUNITY. 527 



it has been grown. It develops readily on the medium that is rich 

 in defibrinated blood, as already described by Gengou and myself 

 in our first article on whooping cough. It may be taught to grow 

 on ordinary agar, in which instance it gives a thick and rather 

 coherent layer. The two varieties of organisms obtained in this 

 manner, although coming from a single original colony, give rise, on 

 immunizing animals, to two different sera. We may consider the 

 serum of a rabbit that has been immunized against the organism 

 grown on ordinary agar. It is found that the serum agglutinates 

 these organisms energetically, but has no clumping effect on a cul- 

 ture of whooping-cough bacillus on the other medium containing 

 defibrinated blood. On the other hand, if we test the serum of a 

 rabbit that has been immunized against the organism grown on 

 blood media, we find that it agglutinates both races of bacteria. A 

 careful study of this phenomena brings out the fact that two definite 

 agglutinins affecting different antigens are present in different pro- 

 portions. One of these antigens, which is present in large amount 

 in the organism that has been developed on blood, is not to be found 

 in the organisms grown on agar. To demonstrate this antigen we 

 take a small amount of the serum and mix it with an excess of bac- 

 teria grown on agar. A few hours later we centrifugalize and 

 decant the supernatant fluid; we find that this fluid no longer 

 affects the same bacteria but has retained entirely its agglutinating 

 effect for the other strain cultivated on defibrinated blood. 



Horses immunized against the organism grown on blood * furnish 

 a serum with an extremely marked agglutinating property for 

 this strain of organism, but practically no active agglutinin for the 

 organism grown on agar. The serum, when treated with even a 

 considerable amount of these latter organisms, even after prolonged 

 contact, is found to have lost none of its specific effect for cultures 

 grown on defibrinated blood. The effect on these latter cultures, 

 then, is due to an antibody which does not find a suitable antigen 

 in bacteria of the same species grown in a different culture medium. 



The two strains that we have considered may be distinguished 

 from other standpoints on which I shall not insist for the moment, 

 particularly as regards their sensitivity to the antibodies of normal 

 sera. I may add, however, that the reaction of alexin fixation 



* Horse blood is used for these cultures. 



