210 



GENERAL BIOLOGY OF MICRO-ORGANISMS 



with the clear filtrate of the cultures of the same bacteria. The 

 parenteral introduction of any foreign protein in solution stimu- 

 lates' the production of a substance which will precipitate it. 1 

 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 vari- 

 ous animals while the milk 

 precipitin is specific, within 

 certain limits, for the one 

 kind of milk. Precipita- 

 tion results only when the 

 blood serum (precipitin) is 

 combined with the proper 

 amount of the culture fil- 

 trate or other protein *so- 



FIG. 86. Receptors of the second order and lution (pretipitinogen) 



some substance uniting with one of them. (Jour- w u pn tnn l pro -p an PYPP^ of 



nal of the American Medical Association, 1905, p. wnen to lar g e an 



1113.) c, Cell receptor of the second order; d, one or the other is used no 

 toxophore or zymophore group of the receptor; 

 e, haptophore of the receptor; /, food substance 



or - r ^ du f / Bacterial disintegration uniting 



the haptophore of the cell receptor. 



-, 



wit 



. . 

 precipitate occurs. 



H h explains 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 stimulates the 

 body cells to produce specific receptors capable of combining 

 with it. In this instance, however, the receptor not only com- 

 bines with the foreign material, but also brings about a definite 

 change in it which is evidenced by the phenomenon of precipita- 

 tion. The side-chain therefore contains at least two distinct 

 atomic groups, one of which serves to combine with the pre- 



1 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., 1912, p. 112. 



