252 Iinmiinity in Some of its Biochemical Aspccts [Jan. 



merely several groups of absolutely immobile bacilli. If the reac- 

 tion is feeble, the clumps are small, and one finds comparatively 

 many isolated and, perhaps, also moving bacteria. This phenome- 

 non is spoken of as aggliitination, and the substance in the serum 

 which brings this about is called agglutinin. The clumping thus 

 broiight about does not kill the bacteria ; moreover, it makes no dif- 

 ference whether the serum is freshly drawn or has been kept for 

 some time — it will agglutinate equally well ; and it does not require 

 the addition of fresh serum as do the bacteriolysins. Like the an- 

 titoxins and the bacteriolysins, the agglutinins are strictly specific, so 

 that serum from an animal previously injected with typhoid bacilli 

 will agglutinate only typhoid bacilli; one from an animal injected 

 with dysentery bacilli, only such bacilli, etc. 



Opsonins. We have already said that the white blood corpus- 

 cles (leucocytes) take up bacteria and destroy them. Wright, of 

 England, showed that certain substances in blood serum have the 

 power of increasing the appetite, as it were, of the leucocytes, and 

 furthermore, that the amount of these substances can be increased 

 by properly graduated injections of the appropriate bacteria. These 

 substances he called Opsonins. They are specific, just as are the 

 antitoxins, the bacteriolysins, and the agglutinins; that is to say, 

 when typhoid bacilli are injected into the body, only the Opsonin 

 for typhoid bacilli is affected; when staphylococci are employed, 

 only the Opsonin for such organisms is affected, etc. 



Precipitins. If, instead of injecting bacteria or other cells, we 

 inject an animal with Solutions of albuminous material ; for example, 

 if we inject a rabbit with chicken-egg albumin, we find that the 

 rabbit serum acquires the power to produce a precipitate when mixed 

 with chicken-egg albumin. This action, too, is highly specific, so 

 that if the serum is tested against the albumin from any other animal, 

 e. g., from a duck tgg, no precipitate will be produced. If a rabbit 

 is treated with human blood, the rabbit serum will produce a pre- 

 cipitate when mixed with human blood, but not when mixed with 

 any other blood. The substance in the treated animal's serum is 

 spoken of as a precipitin. This test, as you probably know, is used 

 in criminal cases to determine whether or not certain stains are 

 those of human blood or otherwise. 



