IMMUNITY AND ANAPHYLAXIS 193 



the latter giving a precipitate. The precipitins are there- 

 fore conceived to be built up of two atom groups, one 

 thermolabile and the other thermostabile. Unlike 

 agglutinins, they have not been, so far, shown to exist in 

 normal serum. This reaction is used in forensic work, 

 to determine the character of blood-stains, whether human 

 or not. 



Opsonic Action. In 1903-4, Sir Almroth Wright 

 undertook a systematic study of the phenomenon of 

 phagocytosis, and showed that phagocytosis depended 

 on a substance present in the serum, which acts on the 

 bacteria (and not on the leucocytes), becomes fixed to 

 them, and makes them a prey to the leucocytes. To this 

 substance he gave the name " opsonin " (feast preparer). 

 This substance is present in normal serum, but can be 

 increased by immunization. It is destroyed by heating 

 to 55 C. Leucocytes washed with salt solution have no 

 phagocytic action. On the other hand, if the bacteria are 

 exposed to the action of serum, and then washed free of it, 

 they can be phagocytozed by the washed leucocytes. It is 

 hence inferred that the opsonin becomes bound to the 

 bacterium. A similar substance has been described in 

 immune sera after heating (bacteriotropin), but it is 

 specific for the corresponding bacterium, while the opsonin 

 in normal serum is non-specific. There are thus two 

 opsonic substances ; that present in normal serum, which is 

 thermolabile, and that present in immune sera, which is 

 thermostabile. In opsonic estimation, both factors are 

 at work where the person is being gradually immunized 

 to a particular bacterium. The whole question is still 

 complex and full of difficulties, and requires further 

 elucidation. 



TECHNIQUE OF OPSONIC ESTIMATION. The method of 

 Leishman is very simple. Take a capillary pipette, fitted 

 with a rubber nipple. Make a mark on the stem ; draw up 

 fresh blood from the finger to the mark. Then draw up 

 a little air to make an air-bubble, which separates the 

 blood from the bacterial emulsion now drawn up to the 

 same mark. The two fluids are then mixed by being 

 blown out on a glass slide, and drawn back repeatedly. 

 Finally, the drop is placed on the slide, covered with a 



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