CH. XXIX.] OPSONINS AND PRECIPITINS 477 



described as the action of an opsonin (derived from a Greek word 

 which means " to prepare the feast " ). 



We may take the specific case of the tubercle bacillus as an 

 instance where such work is of value. All of us are breathing in 

 these bacilli every day of our lives, but many of us escape tubercu- 

 losis because the opsonic power of our blood is sufficiently high to 

 render the bacilli an easy prey to leucocytes. In those to whom the 

 organism is pathogenic, the modern treatment is directed to enhanc- 

 ing nature's cure by increasing the opsonic power of the patient's 

 blood by good food and pure air, or the injection of preparations of 

 the required opsonin. 



Lastly, we come to a question which more directly appeals to the 

 physiologist than the preceding, because experiments in relation to 

 immunity have furnished us with what has hitherto been lacking, a 

 means of distinguishing human blood from the blood of other 

 animals. 



The discovery was made by Tchistovitch (1899), and his original 

 experiment was as follows: Eabbits, dogs, goats, and guinea-pigs 

 were inoculated with eel-serum, which is toxic : he thereby obtained 

 from these animals an antitoxic serum. But the serum was not only 

 antitoxic, but produced a precipitate when added to eel-serum, but 

 not when added to the serum of any other animal. In other words, 

 not only has a specific antitoxin been produced, but also a specific 

 precipitin. Numerous observers have since found that this is a 

 general rule throughout the animal kingdom, including man. If, for 

 instance, a rabbit is treated with human blood, the serum ultimately 

 obtained from the rabbit contains a specific precipitin for human 

 blood; that is to say, a precipitate is formed on adding such a 

 rabbit's serum to human blood, but not on adding it to the blood of 

 any other animal. There may be a slight reaction with the blood 

 of allied animals ; for instance, with monkey's blood in the case of 

 man. The great value of the test is its delicacy ; it will detect the 

 specific blood when it is greatly diluted, after it has been dried for 

 weeks, or even when it is mixed with the blood of other animals. 



The lipoids contained in cells (mainly in the cell-membrane) play some part in 

 the relationship of such cells to toxins. The matter has been mainly studied in 

 relation to red corpuscles, and the hsemolysins (such as snake-venom, saponin, etc.), 

 which attack them. There is some evidence that the cholesterin in the envelope of 

 the red corpuscles is a protective agent (see also p. 435). A few years ago, Preston 

 Kyes stated that lecithin is the amboceptor which anchors the hsemolysin on to 

 the red cells. But more recent research has failed to substantiate this view, and 

 the compounds which Kyes described and called lecithides are impure mixtures 

 of several substances. It is much more probable that the real agent at work 

 in haemoylsis is a lipolytic or fat-splitting enzyme ; this splits up the lecithin of 

 the cell, liberating oleic acid and deoleolecithin (that is, lecithin minus its oleic 

 acid radical), and it is these cleavage products which dissolve out the haemoglobin 

 and so destroy the corpuscles. 



