ANIMAL FLUIDS AND TISSUES. 661 



of the body. They have so far not been of great assistance thera- 

 peutically, but promise to be of great value in the prophylaxis of 

 certain diseases, as in the typhoid vaccination by injections of killed 

 cultures of B. typhosus. 



In this connection it is important to note that bacteria form several 

 classes of toxins or poisons. Ptomaines are produced by the effect 

 of bacteria upon the medium in which they are growing. Bacterial 

 toxins are substances which, in distinction to ptomaines, are elab- 

 orated by the bacteria within the bacterial cells. These are substances 

 of doubtful chemical structure and are divided into two classes : (1) 

 Endotoxins, toxins existing mainly in connection with the bacterial 

 cell, going into solution with difficulty and possessing in such solution 

 only a moderately poisonous action. (2) Soluble toxins, readily re- 

 moved from the bacterial cell by solution and giving a solution of 

 strong toxic action. 



It has been found that an animal which has been treated with bac- 

 teria-producing soluble toxins has present in its blood a specific anti- 

 body which is capable of neutralizing the toxin of the corresponding 

 bacteria. This substance is called an antitoxin and protects, from 

 the poisonous effects of the toxin, not only the original animal (active 

 immunity), but also other animals into which the antitoxin-containing 

 serum may be injected (passive immunity). This production of 

 passive immunity has been widely used in the treatment of diphtheria 

 and the prophylaxis of tetanus. In other bacterial diseases, such as 

 typhoid and streptococcus infections, where the toxins present are 

 mainly endotoxins, the production of an antitoxin conferring passive 

 immunity has not been successful. 



In the case of some of these antibody reactions it is found that a 

 third substance is necessary. This substance is commonly called the 

 complement ; and in this connection the antibody is termed the am- 

 boceptor, and the foreign protein the antigen. The amboceptor and 

 the complement are both present in the blood-serum. Complement 

 is an unstable substance present in all fresh blood, and is not specific, 

 that is, it will enable any amboceptor to act upon the corresponding 

 antigen. Amboceptor is fairly stable, is usually present only in re- 

 sponse to the introduction of some antigen, and is specific, that is, it 

 will act only upon its corresponding antigen. 



As amboceptor will act upon antigen only in the presence of com- 

 plement, the presence or absence of complement in a solution may 

 readily be shown by mixing the solution with a suitable amboceptor 

 and antigen, when the presence of complement is shown by the occur- 



