THEORIES OF IMMUNITY 125 
acts upon foodstuffs after they are anchored to the cell by the com- 
bining 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 inacti\'e 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 
physiological need of the cell into the blood stream. vSide-chains of 
the second order which are thus cast off from the cell in response to 
the stimulation of bacteria 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 
w4th 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 an hour 
or more, 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 tw^o, 
then removed by centrifugalization, it will be found that the bacilli 
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 has 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 rendered 
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 agglvtinoid. It bears a striking resem- 
blance to a toxoid in that the active or ergophore group is destroyed, 
while 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 precipi- 
tinophore group. 
