THEORIES OF IMMUNITY 125 



second order is permanently a part of the structure. The comple- 

 ment, which is analogous to the zymophore group of the second order, 

 is not attached to a side-chain of the third order until the cytophilic 

 group of the latter has combined with its antigen. The zymophore 

 group of the second order side-chain is readily destroyed and it 

 cannot be replaced. The zymophoric group of the third order side- 

 chain is not an integral part of the structure, and it can be introduced 

 under appropriate conditions. 



Third order side-chains or amboceptors are cytolysins. Those 

 specific for bacteria are called bacteriolysins; those specific for blood 

 are called hemolysins; and those specific for the cells of various tissues 

 or organs are called cytolysins. 



The activity of the lysins, according to the Ehrlich theory, depends 

 on the union of non-specific complement and a specific antigen by the 

 specific amboceptor. A union of antigen and amboceptor may take 

 place in the absence of complement, but a union of antigen and com- 

 plement cannot take place in the absence of amboceptor. The 

 amboceptor, like other haptophore groups, is relatively thermostabile. 

 The non-specific complement (found in fresh blood serum from any 

 animal) is thermolabile and readily destroyed. 



Thus far it has been assumed that the cells of the body defend 

 themselves against toxins, alien protein or alien cells by the formation 

 of specific antibodies or side-chains. Welch 1 has made the important 

 suggestion, which has experimental evidence in its favor, that bac- 

 teria may also produce side-chains which are specific for certain 

 cells of the host. A struggle between host and microbe, therefore, 

 would not be one-sided; a dual attempt at immunization is going 

 on during a bacterial invasion, in which the microbe attempts to 

 protect itself against the specific weapons of the host as the host 

 attempts to protect itself against the weapons of the invading micro- 

 organism. Thus, bacteria grown in media containing agglutinating 

 sera gradually lose their agglutinability, but this acquired loss of 

 agglutinating power is not exhibited by descendants of the inagglu- 

 tinable strain 'grown for some time in media not containing agglutinins. 



The side-chain theory, originally formulated to explain antitoxin 

 immunity, but enlarged in its scope to include the phenomena of 

 agglutination, precipitation and cytolysis, has been subjected to much 

 adverse criticism. It was assumed that toxin and antitoxin, for 

 example, united in simple proportions as a strong acid and a strong 



1 Huxley Lecture, 1902. 



