124 IMMUNITY AND INFECTION 



will no longer agglutinate with a fresh, highly potent agglutinating 

 serum. The bacteria are saturated with the combining group of the 

 serum whose agglutinophore group had been inactivated by heating. 

 This experiment shows that the combining group is relatively stabile, 

 and that it is active even though the zymophore group is inactive. 

 A side-chain of the second order which has lost its ability to cause 

 agglutination with a specific organism, but which still retains its 

 combining power, is called an agglutinoid. It bears a striking resem- 

 blance to a toxoid in that the active or ergophore group is destroyed, 

 but the combining group remains intact. 



Sera containing specific precipitins readily lose their ability to form 

 precipitates with the homologous protein. The precipitins have 

 changed to precipitinoids, due to a functional loss of their precipitino- 

 phore group. 



The part played by side chains of the second order, agglutinins and 

 precipitins, in immunity is not well understood. Their relation to 

 immunity is less clear than the relation of antitoxin to immunity. 



Side-chains of the Third Order. Nutritive substances of large mole- 

 cular aggregation may require considerable modification to fit them 

 for cellular assimilation. Such substances are removed from the 

 blood stream and bound to the cell by side-chains of the third order. 

 They are then acted upon by an enzyme (complement) which is 

 also present in the blood stream. It will be seen that both the nutri- 

 tive element and a digestive enzyme circulate in the blood, but that 

 no reaction occurs between them until they are both united by a side- 

 chain of the third order, which must therefore consist essentially of 

 two combining groups. One of these, the cytophilic group or hapto- 

 phore, unites specifically with the nutritive element. The other com- 

 bining or haptophore group, the complementophilic group, unites 

 with the enzyme or complement which is present in the blood stream. 



Side-chains of the third order are called amboceptors because they 

 possess two combining groups. An excessive irritation of a cell by a 

 substance capable of uniting with the cytophilic group of a side-chain 

 of the third order will lead to overproduction and elimination of these 

 side-chains precisely as toxins lead to an overproduction of side-chains 

 of the first order (antitoxin formation). The side-chains of the third 

 order, furthermore, exhibit specificity for the substance which led to 

 their overproduction, just as antitoxins exhibit specificity for their 

 homologous toxins. 



It has been shown that the zymophoric group of a side-chain of the 



