PROTEINS 501 



Of late, it has become possible to render an antigen nontoxic 

 without robbing it of its antigenic capacity to incite antibody 

 formation. Such nontoxic antigens are known as toxoids. For 

 example, diphtheria bacilli can be treated so that they are still 

 antigenic, i.e., still have the capacity to cause antibody produc- 

 tion, but are now nontoxic and so cannot cause diphtheria. The 

 toxin is now a toxoid. Toxoids may be injected directly into 

 human beings without the intermediary of an animal, because 

 while we ourselves are unable to produce a diphtheria antitoxin 

 in sufficient quantity fully to counteract the toxin when attacked 

 by the disease directly, we can and do produce antitoxin ade- 

 quately when injected with toxoid. 



Most immunization is for the purpose of prevention rather than 

 cure; thus, vaccination for smallpox and inoculation for typhoid 

 are done to healthy people in anticipation of the disease. The 

 treatment is prophylactic. Once within cells, the disease-pro- 

 ducing parasites cannot be easily reached by injected serums, 

 while if the prophylactic serum is already present in the body 

 fluids, then the parasites will be killed when they enter the body, 

 as they are intercellular before they can become intracellular. 

 Some few antitoxins, such as those against diphtheria and tetanus, 

 act as cures but only when administered in the very early stages 

 of illness. 



Human sensitivity to foreign proteins, such as those in the 

 serum of horse blood, is an example of that important branch of 

 protein individuality known as allergy. 



We come now to a more detailed consideration of the question 

 how far immunity and allergy are problems in protein chemistry; 

 in other words, are the proteins of the cell and of the body fluids 

 the responsible substances? 



It was formerly thought that no substance other than a protein 

 could be an antigen and that, on the other hand, nearly every 

 known protein is antigenic (to some animal). Thus, Wells wrote 

 that immunological specificity must depend in large part upon 

 differences in proteins, for antigens a,re usually, if not always, 

 proteins. While Wells realized that other substances might on 

 occasion be responsible, he added that if we turn to one of the 

 most likely of such other substances, the lipoids, we find Levene 

 saying that he has failed to discover any distinction between 

 lipoids derived from different tissues of the same species. On 



