CHEMICAL ACHIEVEMENT — PAULING 233 



polysaccharide, either pure or part of the structure of an animal 

 or plant cell — is injected into an animal, such as a rabbit, the animal 

 in the course of a few days may develop in its blood and within its 

 cells substances called antibodies which have the power of specific 

 combination with the injected material, the antigen. Thus, when a 

 particular animal or plant protein is injected into a rabbit, the rabbit 

 develops in its blood antibodies which are capable of combining with 

 that protein, but not, or at any rate only very exceptionally, capable 

 of combining with any of the tens of thousands of other proteins 

 which exist in nature. For example, an antiserum made by injecting 

 hemoglobin obtained from one animal into a rabbit is able to combine 

 with that form of hemoglobin, but not with hemoglobin obtained 

 from the red cells of other animals, except those of very closely re- 

 lated species. The act of combination of antibody and its homologous 

 antigen may be shown by several different phenomena, such as the 

 agglutination of cells, in the case of a cellular antigen, the formation 

 of a precipitate on mixing a solution of antigen and its homologous 

 antibody, the allergic response of a sensitized animal on receiving 

 a subsequent injection of the antigen, and the lysis or other changed 

 behavior of cells to which antibody has attached itself. 



The phenomena of immunochemistry raise two great questions. 

 The first concerns the nature of the forces between antibody and 

 antigen, which lead to the power of selective combination of anti- 

 body and the homologous antigen and the rejection of other molecules, 

 except those very closely related to the homologous antigen. The 

 second problem is that of the mechanism of the manufacture of the 

 antibody, and of its endowment with this power of specific 

 combination. 



The great versatility of living organisms in their production of 

 specific antibodies was shown by the early work of Landsteiner with 

 artificial conjugated proteins as antigens. Landsteiner found that 

 it was possible to cause an animal to make antibodies with the power 

 of specific combination with various chemical substances of known 

 structure. He achieved this by attaching these chemical substances 

 to a protein molecule, which was then injected into a rabbit. The 

 rabbit, under the influence of the injected protein, produced an anti- 

 serum containing antibodies capable in general of combining with 

 the particular protein that was used in making the artificial conju- 

 gated protein, and also capable of combining with the attached 

 chemical substances. For example, an antiserum prepared by cou- 

 pling diazotized ;?-aminobenzenearsonic acid with ovalbumin was 

 found to form a precipitate with this particular azoprotein, and in 

 addition to precipitate, in smaller amounts, ovalbumin itself and also 

 any azoprotein made by coupling diazotized p-aminobenzenearsonic 



