THEORIES OF IMMUNITY 123 



they are bound to the cell. A side-chain of the first order, which 

 possesses simply a combining group, does not provide the requisite 

 power of digestion, according to Ehrlich, and to effect this digestion 

 side-chains of somewhat more complex structure are required. Side- 

 chains of this more complex type, side-chains of the second order, 

 possess not only a combining group for the foodstuff, but a digestive 

 group as well. This digestive or zymophore group, as it is called, acts 

 upon foodstuffs after they are anchored to the cell by the combining 

 or haptophore group. The complete side-chain of the second order, 

 therefore, is composed essentially of a combining or haptophore group, 

 and a zymophore group as well. The haptophore group of the second 

 order side-chain is relatively stabile, but the zymophore group is 

 labile and readily becomes inactive without, however, impairing the 

 original combining ability of the side-chain. Side-chains of the second 

 order are as vulnerable to pathological substances possessing the 

 requisite chemical affinity as side-chains of the first order, and repeated 

 irritation of a cell by such pathological substances leads eventually 

 to an overproduction of side-chains of the second order and an 

 elimination of the supernumerary side-chains in excess of the physio- 

 logical need of the cell into the blood stream. Side-chains of the 

 second order which are thus cast off from the cell in response to the 

 stimulation of bacterial or other alien protein are of importance 

 immunologically. If the serum of an animal containing such side- 

 chains is brought into contact with a suspension of the homologous 

 bacterium, the organisms are sooner or later clumped together or 

 agglutinated. If, on the contrary, the serum is brought into contact 

 with a clear solution of the homologous protein, a precipitate forms. 

 These reactions are highly specific and those side-chains which cause 

 agglutination of the specific bacterium or precipitation with the 

 homologous protein solution are called respectively, agglutinins and 

 precipitins. 



The relative instability of a zymophore group of a side-chain of the 

 second order may be inferred from the following experiment : 



A serum obtained by injecting a horse with repeated graduated 

 doses of typhoid bacilli will clump or agglutinate the specific organism 

 in high dilution. If the serum is heated to 60 or 70 C. for a few 

 minutes, or if it has been kept for a long time, it will no longer clump 

 the bacilli, or, at least, it will clump them imperfectly. If such a serum 

 is allowed to stand in contact with typhoid bacilli for an hour or two 

 then removed by centrifugalization, it will be found that the bacilli 



