8 June, 1908.] Elements of Animal Physiology. 351 



IMMUNITY. — Whenever a cell, a protein, or a ferment, foreign to 

 the tody, enters the blood, it provokes the formation of a neutralizirig or 

 antagonistic substance. The exact site of origin of these substances is 

 not fully understood, one school regarding the white cells as the sole 

 source, another school asserting that the tissues affected by the foreign 

 substance supply the antibody. These antagonistic substances are of 

 various sorts and can be briefly enumerated as follows: — 



1. Foreign proteins excite the formation of frecifitins, which when 



mixed with the exciting protein produce precipitates. Thus 

 if white of egg is injected several times into a rabbit the blood 

 of this animal will contain a precipitin which can readily be 

 demonstrated by mixing egg white with the rabbit's serum. 

 These precipitins like all the other antibodies are fairly specific. 

 For instance the precipitin evoked bv the albumen of a hen's 

 egg will give a well defined precipitate with the white of a 

 hen's egg but none or hardlv anv with the white of a duck's 

 egg. The proteins of man's blood will excite in the same 

 way a precipitin which reacts best with a protein derived 

 from a man or anthropoid ape. 



2. Enzvmes excite the fonnation of anti enzymes. Even the digestive 



ferments in the alimentarv canal of an animal provoke in 

 the same animal the formation of antitrvpsin, &c., which fact 

 partly explains why these digestive ferments act only on the 

 food and not on the lining walls of the gut. 



3. The toxins which bacteria produce and which can gain admittance 



to the general circulation, giving rise to various disturbances, 

 excite the formation of antitoxins which neutralise these 

 bacterial products. 



4. Foreign cells, including bacteria and foreign blood corpuscles, 



excite the formation of a number of antagonistic bodies — 



{a) Agghiiinins, which cause the foreign cells to clump 



together, 

 {b) Opsonins which so act upon bacteria, &c., that these 



latter are readilv devoured by white cells, 

 (c) Cytoioxins which break up the cell-wall and cause dis- 

 integration of the cell. Thus the red copuscles of a 

 sheep, injected into a rabbit, will be followed by the 

 formation in the rabbit of a substance which destroys 

 the envelope of the red corpuscles of the sheep and 

 acts only in a feeble manner on the red corpuscles of 

 other animals. 

 When an animal acquires a disease and, after its recovery, is no longer 

 liable to contract the same disease, as occurs, for instance, with typhoid 

 fever in man and distemper in dogs, the immunity acquired is explained 

 by assuming that these antibodies, formed during the disease, remain in 

 the circulation and effectually prevent each new invasion from making any 

 headway. No substances sirnpler than the proteins or the enzvmes appear 

 capable of exciting the formation of antibodies ; thus ricin, the poisonous 

 protein of castor-oil seeds, causes the production of antiricin, but alkaloids 

 such as morphia or strychnine or other and simpler substances cannot art 

 in this way. 



The total amount of lilood in the bmlv is abo-ut one-fifteenth to 

 one-twentieth of the total weight of the animal. 



