22 THE PRINCIPLES OF IMMUNOLOGY 



Artificially Acquired Immunity. The artificial acquisition of im- 

 munity may be the result of active development of immune sub- 

 stances in the organism or it may be due to the transfer into the 

 organism of immune substances from an immune animal. Arti- 

 ficially acquired immunity differs from naturally acquired immunity 

 in that it is likely to be less durable. If acquired by active immuniza- 

 tion the duration is likely to be considerably greater than if acquired 

 by passive immunization. In the discussion of immunity it is well 

 to keep clearly in mind the definition of antigen and antibody. The 

 antigen is a substance which upon introduction into the body in 

 proper amounts and under suitable conditions induces the formation 

 of a special antagonistic substance, the antibody. Conversely the 

 antibody is the substance produced as a result of the introduction 

 of antigen. Experimentally the antigen is usually introduced by 

 parenteral routes, meaning routes other than by way of the ali- 

 mentary canal, such as intravenous, intraperitoneal, subcutaneous, 

 intrathecal, intraocular, and by other similar pathways. The nature 

 of antigens and antibodies will be discussed in the subsequent chap- 

 ters, but it may be said here that both are of protein nature. Every 

 soluble complete protein, with the exception of the racemized pro- 

 tein of Dakin, may serve in at least some degree as an antigen. 

 The proteins employed are for the most part native, but synthetic 

 proteins may also act as antigens. Wells states that " of the cleav- 

 age products of proteins it is certain that none of the amino-acids 

 and simple polypeptids can act as antigens, and it is not yet fully 

 established that even such large complexes as the proteoses are 

 antigenic, although there is some evidence in favor of this view." 

 There have been numerous reports of the use of lipoids as antigens, 

 but this relation has not been definitely established. If lipoids are 

 obtained from animal tissues favorable results may be obtained, but 

 in none of these experiments is it proven that the lipoids are entirely 

 free from proteins. Ford has successfully employed a hemolytic 

 glucoside obtained from the poisonous mushroom amanita phalloides 

 as an antigen for the production of an anti-hemolysin, but this is 

 the only well-established exception to the general rule that antigens 

 are of protein nature. 



Actively Acquired Immunity. This may be produced by actual 

 infection of an individual during a period of good health by the 

 virus of the disease to which he is to be immunized. The classical 

 example of this form of immunization was the practice for many 

 centuries of inoculating smallpox into the healthy, so as to induce 

 a mild attack of the disease. The danger lies in the uncertainty 

 of action of the virus, since apparent health does not necessarily 

 presuppose resistance to any special disease. If the virus can be 

 measured in some way so that an extremely small amount can be 

 inoculated, the procedure is somewhat safer. Protection against Texas 

 fever in cattle has been practised by permitting nursing calves to be 



