IMMUNITY 195 



human blood, less so with ape's blood, not at all with other blood ; 

 ox blood anti-serum reacts with ox blood, less so with sheep, feebly 

 with horse, hardly at all with dog. Mixtures of bloods may also 

 be tested. Precipitins are also formed naturally in vivo. Thus 

 the serum of a patient the subject of hydatid disease gives a precipi- 

 tate with hydatid fluid, and the reaction may be used diagnostically. 

 The production of the anti-body seems to be due to the globulin 

 constituent of the injected serum. 



It will thus be seen that the anti-bodies which result 

 from the injection into an animal of different substances 

 are extremely numerous and have varied properties, their 

 most notable characteristics being their extreme specificity 

 and the extraordinary delicacy of the interactions produced 

 by them. It is important to note that these anti-bodies 

 are produced only as the result of inoculation with complex 

 compounds allied to the proteins. The tolerance estab- 

 lished by the ingestion or inoculation of simpler com- 

 pounds, such as arsenious acid and morphine, is of a different 

 nature, and is not coincident with the development of 

 anti-bodies. According to Ehrlich, the latter kind of 

 tolerance may be due to the exhaustion or using up of 

 certain receptors (" chemo-receptors ") of the protoplasm 

 (see p. 206). 



Immunity * 



No fact in biology is more striking than the differences 

 in susceptibility to infection exhibited by different races 

 and different animals. For example, the natives in many 

 parts of the world are comparatively insusceptible to yellow 

 and typhoid fevers and malaria, the dog and goat are rarely 

 affected with tuberculosis, and tetanus is never met with 

 in the fowl ; and to come nearer home, while some indi- 

 viduals are lucky enough to escape most of the commoner 



1 See Metchnikoff, Immunity in Infective Diseases, 1905. Also Brit. 

 Med. Journ., 1902, vol. i, p. 784 ; 1904, vol. ii, pp. 557-582 ; and 1907, 

 vol. ii, pp. 1409-1425 ; Journ. of Hygiene, vol. ii, 1902 ; Emery, Im- 

 munity and Specific Therapy, 1909. 



