REACTION OF THE HOST TO INFECTION 



217 



lates the production of a substance which will precipitate it.^ 

 These substances, which are called precipitins, resemble enzymes 

 in many respects. Thus, the precipitin produced by the injec- 

 tion of a milk, causes a change in the milk very similar to that 

 caused by rennet. Rennet, however, coagulates milk from various 

 animals while the milk pre- 

 cipitin is specific, within cer- 

 tain limits, for the one kind 

 of milk. Precipitation re- 

 sults only when the blood 

 serum (precipitin) is com- 

 bined with the proper 

 amount of the culture fil- 

 trate or other protein solu- 

 tion (precipitinogen)- — when 

 too large an excess of one or 

 the other is used n,o precipi- 

 tate occurs. Ehrlich ex- 

 plains the formation of 

 precipitins on the basis of 

 his side-chain theory in the 

 same way as the production 

 of antitoxins was explained. 

 The foreign protein stimu- 

 lates the body cells to pro- 

 duce specific receptors 



capable of combining with it.' In this instance, however, the re- 

 ceptor not only combines with the foreign material, but also brings 

 about a definite change in it which is evidenced by the phenomenon 

 of precipitation. The side-chain therefore contains at least two 

 distinct atomic groups, one of which serves to combine with the 

 precipitinogen, and is specific in nature, and another which brings 



^ Specific precipitin tests have been employed to some extent in determining 

 the source of blood stains and of meats. See Citron, Immunity, translated by 

 Garbat, Phila., 1914, p- 125- 



Fig. 88. — Receptors of the second order 

 and some substance uniting with one of 

 them. (Journal of the American Medical 

 Association, 1905, p. 1113.) c. Cell receptor 

 of the second order; d, toxophore or zymo- 

 phore group of the receptor; e, haptophore 

 of the receptor; /, food substance or product 

 of bacterial disintegration uniting with the 

 haptophore of the cell receptor. 



