TERMINOLOGY 195 



administered via the alimentary canal are either not absorbed, or are partly or 

 wholly disintegrated by the digestive fluids. 



It will be noted that an antigen, as defined, has two essential attributes. It must 

 react with the specific antibody, i.e. the antibody produced in response to its 

 entrance into the tissues ; and it must have the power of stimulating the production 

 of that antibody. Examples will later be noted of substances which react 

 specifically with a given antibody, without possessing the power of stimulating its 

 formation. Such substances are not true and complete antigens, though they are 

 closely related to this class by the property of specific reaction. 



The term antibody, as defined, has also two attributes, that of specific reaction 

 with the antigen, and that of being produced in response to the stimulus provided 

 by the access of that antigen to the tissues. In practice it is not customary to 

 insist on the demonstration of the second attribute. When we prepare an anti- 

 serum by inoculation, or find antibodies in the serum of a man or animal suffering 

 from a particular bacterial infection, the condition of specific stimulation may be 

 assumed to have been fulfilled. But there are numerous cases in which we find, 

 in the blood of normal and untreated animals, substances which react with various 

 foreign cells or bacteria in ways which are essentially similar to those observed 

 when we mix an antigen with the specific antibody which has been produced 

 by artificial immunization. It is usual to refer to these substances as " natural " 

 or " normal " antibodies. They may, in many cases, have arisen as the result 

 of natural infection ; in others they are probably an expression of the chemical 

 interrelationships between living organisms, which have arisen during the long 

 course of evolution (see Chapter 50). 



Having decided upon a generic term for each of the two essential reagents, it 

 became necessary to name in some way the particular substances which were 

 assumed to be involved in the different reactions which were observed. Little 

 being known of their nature, they were named according to what they did, instead 

 of according to what they were. The names given described the reaction observed 

 when they were mixed with the corresponding antigens, the suffix -in being added 

 to the descriptive term. Thus, an antibody which caused lysis was called a lysin, 

 a hcemolysin if it acted upon red blood corpuscles, a bacteriolysin if it acted ujDon 

 bacteria. An antibody which gave rise to agglutination was called an agglutinin, 

 an antibody which caused precipitation a precipitin, and so on. 



The actual material, organized or unorganized, with which these antibodies 

 reacted, was in general provided with some well-recognized name ; so that one 

 could speak of bacteria, or of red blood corpuscles, or of foreign proteins, as 

 antigens. But it was clearly realized that many of these antigenic materials were 

 highly complex, and that the antigenic property was almost certainly confined to 

 some particular part of the material in question. It thus became customary to 

 use a term, derived from the name of the antibody, to denote the essential part 

 of the antigenic material which functioned as a stimulus to antibody production 

 and reacted specifically with the antibody so formed. This was accomplished by 

 adding the suffix -ogen to the name of the corresponding antibody. 



Thus bacteria, when injected into the tissues, act as antigens and stimulate 

 the production of agglutinins ; but the particular part of the bacterial substance 

 which provides the specific stimulus is spoken of as an agglutinogen and one 

 bacterium may possess several different agglutinogens. 



Although the terms agglutinogen, precipitinogen, etc., were in common use until 



