444 THE BLOOD [CH. XXVI. 



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 when added 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. 



We thus see that the means of defence in the body are numerous. In some cases 

 bacteria are killed by the bacteriolysins of the blood ; in other cases the toxins the 

 bacteria produce are neutralized by antitoxins. In other cases still the bacteria are 

 directly attacked and devoured by the white corpuscles or phagocytes. In connec- 

 tion with phagocytosis great differences are noticeable ; this is partly explained by 

 what is called chemotaxis ; some bacteria produce chemical substances that attract 

 the leucocytes to their neighbourhood (position chemotaxis) : in other cases, such 

 chemical magnets are not produced, or even negative chemotaxis may occur and the 

 phagocytes be repelled. The recent discovery of opsonins by A. E. Wright is in this 

 connection one of great importance ; it illustrates how the body fluids and the leuco- 

 cytes co-operate, and further shows how elaborate are the means the body possesses 

 for combating bacterial invasion. The word opsonin is derived from a Greek word 

 which means " to prepare the feast" Washed bacteria from a culture are distasteful 

 to phagocytes ; but if the bacteria have been previously soaked in serum, especially 

 if that serum has been obtained from the blood of an animal previously immunised 

 against that special bacterium, then the leucocytes devour them eagerly ; in other 

 words, something has been added to the bacterium to make it tasty, and each kind 

 of bacterium apparently requires its own special sauce or opsonin. 



Heematoporphyrin (see p. 431). If oxyhaemoglobin is treated with dilute 

 acids the result is a formation of haematin and globin, but if strong sulphuric acid is 

 employed the iron is removed from the haematin and so haematoporphyrin is 

 obtained. The stability of the iron in the molecule is due to the presence of oxygen, 

 for with the reduced pigment, haematoporphyrin is obtained even when dilute acids 

 are employed. Pure haematoporphyrin can once more be converted into haematin 

 (that is, the iron can be replaced) by warming a solution in dilute ammonia and adding 

 a little Stokes' fluid, and a few drops of a reducing agent like hydrazine hydrate. 

 If cuprammonium solution is used instead of Stokes' fluid in this experiment, a 

 copper compound of haematoporphyrin is obtained, which is identical with turacin, 

 the bright red copper- containing pigment found in the plumage of the plantain- 

 eating birds. (Laidlaw.) 



