7o6 ANTIGENS AND THEIR SPECIFICITY 



activation with acid and alkali, toxins behave Uke peptids in which acids transform 

 cycHc groups to open chains, and alkaHes restore the cyclic structure.' 



It is possible that toxins consist of toxic radicals attached to proteins, and that 

 the purest toxins, which do not give chemical tests for proteins, represent the toxic 

 radical stripped more or less completely away from the proteins with which it is 

 usually bound. Such isolated radicals could readily reunite with the proteins of an 

 animal into which the toxin was injected, and the resulting complex serve as antigen. 

 Such formation of specific antigenic proteins by union of a protein with non- 

 protein radicals is a well-known phenomenon. In this way is explained the fact that 

 people sometimes exhibit sensitivity to non-protein substances, such as iodin, 

 formaldehyde, salicylic acid, and other chemicals which are capable of uniting with 

 an individual's own proteins, thus converting them into foreign proteins which serve 

 as antigens. It is possible, therefore, that toxins are proteins to which highly toxic 

 radicals are attached, which radicals may or may not be an essential part of the 

 protein molecule. This possibility is supported by the fact that the venoms of differ- 

 ent snakes of a single group (e.g., the vipers) may produce identical physiological and 

 anatomical effects, which suggests that the poisonous elements of each is the same; 

 but the immune serum against each venom shows specific differences, indicating that 

 the protein radicals are different for each species. Landsteiner has also shown that 

 proteins may be combined with various organic radicals, so that the compound 

 proteins act as antigens which produce antibodies reacting with any sort of protem to 

 which the same or similar chemical groups are attached. 



NON-PROTEIN ANTIGENS 



The existence of non-protein antigens is still an unsettled question. From time 

 to time reports have been made of successful immunization with materials believed 

 to be free from proteins. Lipoidal mixtures^ as obtained from tissues by extraction 

 have been especially mentioned as non-protein antigens, although attempts to secure 

 antigenic action with thoroughly purified lipoids have been unsuccessful. There are 

 two chief difficulties in accepting the idea that pure lipoids can serve as antigens. 



First, an antigen must be a substance sufficiently foreign to the injected animal to 

 stimulate the defense reaction of antibody formation. As far as our chemical evidence 

 goes, the lipoids of animals are very few in number, of relatively simple structure, and 

 apparently quite the same among widespread species. The proteins and even the 

 carbohydrates exhibit infinitely greater variation, and proteins from one species have 

 in some instances been demonstrated to be chemically distinct from proteins of sunilar 

 function in another species. It is difficult to understand how a lipoid injected into an 

 animal whose tissues already contain the same sort of lipoid could incite the formation 

 of antibodies against this lipoid. If such antibodies were formed they would be ex- 

 pected to have disastrous effects on the lipoids and tissues of the immunized animal 

 itself. 



Second, it is not easily possible to secure lipoid extracts free from proteins, and 



' Hallauer, C: Zlschr.f. Ilyg. ii. Infckliunskrankh., 105, 138. 1925; 106, 253. 1926. 

 - For a presentation of the view that hpoids are important as antigens, sec Much, H.: Virchow's 

 Arch.f. path. Anal., 246, 292. 1923; Miinch. med. Wchnschr., 72, 2089. 1925. 



