2o6 BIOLOGICAL CHEMISTRY 



These various substances are extremely specific. The 

 agglutinins are used for diagnostic purposes. Suppose an 

 individual is suspected to be infected with typhoid bacteria. 

 A sample of his blood serum is diluted and mixed with a 

 suspension of a culture of typhoid bacillus. If the individual 

 has or has had typhoid fever the bacteria are caused to clump, 

 but if not they remain separated and in active movement. 



The precipitin test is used for medico-legal purposes and for 

 the determination of the relationships of species. Blood 

 serum of the human species is injected on several different 

 occasions into a rabbit. After this has been done it is found 

 that the blood serum of this rabbit will cause a precipitate 

 with human blood serum even if the former is diluted. The 

 serum from the same rabbit will not give a precipitate with 

 the serum of the sheep, ox, etc. In a case of suspected murder 

 blood stains can be examined to see if they are from human 

 blood or that of domestic animals. 



The production of antitoxins, agglutinins and opsonins is 

 the underlying object in the production of immunity by 

 bacterial vaccines. An emulsion of dead bacteria injected 

 into the body tissues causes a reaction whereby the power is 

 increased of resisting the same species of bacteria that were 

 used for the inoculation. 



The serum of a rabbit immunised to human serum may 

 show a slight precipitate with the serum of higher apes. 

 The degree of dilution at which the rabbit's serum can still 

 give this precipitate shows the nearness of kin in the different 

 species, and this method has been used to show the relationship 

 of various species to each other.* 



The striking point about these reactions is the high degree of 

 specificity. We cannot distinguish, by ordinary chemical 

 means, between the blood serum of different animals, but in 

 the precipitin test we have a means of distinguishing the 

 various sera with extremely small amounts of material. 



Some time after the injection of a foreign serum a stage 

 results in which a second injection of even a small amount of 

 serum from the same foreign species is followed by collapse, 

 and even death. This is called anaphylactic shock and it is 

 extremely undesirable when using serum treatment, f The 

 anaphylactic condition can be prevented by injecting an 

 extremely small dose of serum ; this removes the anaphylactic 

 condition without bad results, after which larger doses can be 



* G. H. F. Nuttall, Proc. Roy. Soc., 1901, vol. 69, p. 150. 

 f M. Portier and C. Kichet, Soc. Biul., 1902, vol. 54, p. 170. 



