252 INFECTION AND IMMUNITY 



The macroscopic agglutination test, always preferable for exact 

 laboratory researcli, is made in narrow test tubes especially designed 

 for the purpose, measuring about 0.5 cm. in diameter and about 5 cm. in 

 length (Fig. 60, p. 230). 



In these test tubes equal quantities, usually 1 c.c. each, of serum 

 dilution and emulsion are mixed. A series of tubes is prepared, in each 

 subsequent one of which the dilution is higher. These mixtures may 

 be placed in the incubator for a few hours and then kept at room tem- 

 perature. It has been observed by Hiss that after removal from the in- 

 cubator agglutination is in some instances hastened by transference to 

 the ice chest. When agglutination takes place in these tubes, clumps of 

 bacteria may be seen to form, which settle to the bottom of the tube, very 

 much like snow-flakes. The surface of the sediment is heaped up and 

 irregular. The supernatant fluid becomes entirely clear. When the 

 reaction does not occur the sediment is an even, granular one with a flat 

 surface, and the emulsion remains turbid. 



Instead of using test tubes as described above, Wright has sug- 

 gested the use of throttle pipettes of comparatively large diameter into 

 each of which at least three or four different dilutions can be sucked 

 with a nipple, a small air bubble being left between the mixtures. By 

 sealing the distal end of these pipettes in a flame the various dilutions 

 are kept at a distance from each other, and the pipettes may be set on 

 end in a tumbler and observed just as are the test tubes (Fig. 68, p. 285). 



Precipitin Tests. — In an earlier section on precipitins we have seen 

 that precipitates are formed when clear filtrates of bacterial extracts 

 or of broth cultures are mixed with their specific immune sera. 

 Such precipitin reactions are not limited to the realm of bacteria, but 

 have a broad biological significance, in that specific precipitating sera 

 may be produced with proteids of varied source. 



For actually carrying out a precipitin test, the following reagents are 

 required : 



1. A specific precipitating antiserum (antibacterial or antiproteid) ; 



2. A bacterial filtrate or proteid solution. 



The Production of Precipitating Antisera.' — Antibacterial 

 precipitins may be produced in animals by any one of a variety of 

 methods. Animals, preferably rabbits, are injected either with broth 

 cultures or with salt solution emulsions of agar cultures of the bacteria, 

 in gradually increasing quantities. Five or six injections are given in- 



' R. Kraus, Wien. klin. Woch., 1897; Norris, Jour. Inf. Dis., 1 and 3, 1904. 



