30 THE PRINCIPLES OF IMMUNOLOGY 



range of possibility in this connection is indicated in the building- 

 stone theory of Abderhalden. Bearing in mind that practically all 

 immune substances are protein in nature and that proteins are made 

 up of numerous amino-acids, Abderhalden calculated that twenty 

 amino-acids could be so combined as to form 2,432,902,008,176,640,000 

 different compounds. He illustrates this possibility by stating that 

 if three amino-acids are building stones which may be designated 

 A, B, and C, they can be grouped together so as to form six different 

 combinations, ABC, ACB, BCA, BAC, CAB, and CBA, and that 

 four building stones can form twenty-six such combinations and so 

 on until the enormous possibility of different combinations of 

 twenty amino-acids is reached, as illustrated in the figures given 

 above. Chemically no such enormous number of proteins is known, 

 but if immune specificity could be shown to depend upon slight 

 differences of molecular arrangement, Abderhalden's figures indi- 

 cate the number of immunologically specific proteins obtainable. 

 Taking for granted the phenomenon of specificity, that of the group 

 reactions can be more readily explained. In this case it is assumed 

 that in the proteins of closely related species there is some group of 

 molecules common to these species, and further that the formation 

 of immune substances in response to injections of this common 

 group leads to the production of a substance which may react with 

 the common group. In each species, however, there is in addition 

 to the common group special groups which determine the specificity 

 of the substance as an antigen as well as the production of an im- 

 mune substance with a higher degree of affinity for the combined 

 groups of the particular species than for the common group. 



Non-specific Therapy of Infectious Disease. As a result of the 

 extensive studies of infectious disease various modes of treatment 

 have been elaborated. It is well understood that the organism 

 offers resistance to these infections and that the support of circula- 

 tion and excretion by simpler pharmacological methods aids mate- 

 rially in the treatment. Not only is this true, but the investigation 

 of various drugs has determined the specific chemo-therapeutic treat- 

 ment of infections. Examples of this are seen in the use of quinine 

 in malaria, arsenic in trypanosomiasis and spirochetosis, and of 

 emetin in amebiasis. The treatment based more particularly upon 

 immunological methods has been largely specific, but more recent 

 studies have given encouragement in the use of certain non-specific 

 methods of treatment. It was found, for example, that the use of 

 typhoid vaccine is of value not only in the treatment of typhoid 

 fever, but in other diseases, and typhoid vaccines either in the form 

 of the usual killed organisms or organisms sensitized with specific 

 immune sera have produced beneficial results in such diseases as 

 acute articular rheumatism, sub-acute and chronic arthritis, and in 

 certain other infections. Similarly the use of blood serum, of pure 

 proteins, of leucocyte extracts, of fibrin derivatives, and of certain 

 other protein derivatives has appeared to be beneficial. It is not to 



