IMMUNITY 239 



anti-bodies are produced only as the result of inoculation 

 with complex compounds allied to the proteins. The 

 tolerance established by the ingestion or inoculation of 

 simpler compounds, 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. 253). 



Immunity r 



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, animals do not 

 suffer from typhoid fever or cholera, and tetanus is never 

 met with in the fowl ; and, to come nearer home, while 

 some individuals are lucky enough to escape most of the 

 commoner infectious fevers, others seem to contract them 

 on every possible occasion. These instances show that 

 there is often a natural insusceptibility to infective 

 disease, or a natural immunity, as it is termed. This 

 may be complete or partial, or it may appertain only to 

 a race " racial immunity " ; or, varying in different 

 individuals and at different ages, it constitutes " in- 

 dividual immunity," as in the case of diphtheria and 

 scarlatina, which become more and more rare as age 

 advances. 



Still more remarkable, perhaps, is the fact that an insus- 

 ceptibility may be acquired after an attack of infective 



1 See Metchnikoff, Immunity in Infective Diseases, 1905 ; Emery, Im- 

 munity and Specific Therapy. 1909 ; Bordet, L^Immunite dans les Maladies 

 Infectieuses, 1920 (Masson & Cie.). 



