SERUM M ACTIVE IMMUNITY. 167 



inoculation of killed cultures or bacterial products 

 (e. g., staphylococci, tuberculin) the attempt is 

 definitely made to increase the opsonins and other 

 antibodies in the patient's blood. 



One may ask if acquired immunity to bacteria The serum 



. , , ,-, in Active 



and to toxins is due to the presence of the anti- immunity. 

 bacterial and antitoxic substances which were 

 mentioned in connection with natural immunity. 

 Although normal serum is strongly bactericidal 

 for the typhoid bacillus, the serum of one who has 

 recovered from typhoid fever possesses this power 

 to a much greater degree. As this is true in many 

 other bacterial infections, the new resistance is 

 held to depend on the increase of bactericidal sub- 

 stances in the serum. Similarly in acquired im- 

 munity to diphtheria and to tetanus, the most 

 conspicuous change is a great increase in the cor- 

 responding antitoxins. The result is the same, 

 regardless of whether the immunity be produced 

 by a natural attack of the disease, or by artificial 

 immunization with the specific microbe or toxin. 

 Accordingly it seems probable that acquired im- 

 munity in these instances depends on the presence 

 in the serum of an increased amount of properties 

 which, to a certain degree, may be present nor- 

 mally. On the other hand, acquired immunity is 

 not always represented by an increase in the bac- 

 tericidal or antitoxic power of the serum. Bac- 

 tericidal antibodies may, indeed, be formed, but, if 

 so, the micro-organisms concerned are not sus- 

 ceptible to their action. 1 This is the case with the 



1. By the method of complement fixation, which will be 

 explained later, it has indeed been shown that practically all 

 organisms are able to cause the formation of some type of 

 antibody. 



