206 METHODS OF SERO-DIAGNOSIS. 



of the subject for examination, then estimate the average number 

 of bacteria ingested by one leucocyte in this mixture, and make 

 a similar determination in the case of the mixture containing the 

 serum of the normal individuals. The average number found 

 from the former calculation divided by that found in the latter 

 instance indicates the opsonic index of the subject. 



It may be mentioned here, before we pass on to see further 

 how the presence of various antibodies may be utilised to establish 

 a diagnosis of certain infections, that the action of these anti- 

 bodies cannot be said to be absolutely specific. It is well known, 

 for instance, that agglutinins, lysins, and precipitins, may exert 

 their action not alone on the antigens against which they are 

 prepared, but also on those of closely-allied origin. 



Thus a serum hemolytic for the corpuscles of the horse or 

 sheep may also act on the ass or goat in the respective instances, 

 or a serum agglutinating one species of bacterium, as, for instance, 

 the typhoid bacillus, may also agglutinate members of closelv- 

 ailied species such as the bacillus coli, and paratyphoid organisms 

 this constituting the so-called group re-action. Similarly, a 

 prepared serum causing a precipitate with the serum of an animal 

 like the rabbit, may also cause precipitate formation with serum 

 of a hare. 



It is the case however, that the most marked reaction occurs 

 between the antibody and the specific antigen producing it and 

 we minimise to the greatest extent possible the error due to 

 group reactions by bringing the antibody into contact with the 

 antigen under conditions of marked dilution, or sometimes by 

 using an absorption method. 



It must also be said that sometimes, in utilising sero-diag- 

 nosis, cases are met with where the serum of the normal individual 

 may give certain reactions, as, for instance, in the case of the 

 serum of the horse, which, normally, has an agglutinating effect 

 on the bacillus of glanders, which only disappear from evidence 

 on dilution of the serum. 



The practical value of this method of diagnosis is evident in 

 the case of those diseases where a direct demonstration of the 

 causal organism of the injection may either be impossible, or 

 possible only with difficulty at the time of the examination of 

 the animal, as in cases of occult glanders in the horse; or con- 

 tagious abortion in cattle; or infection with the micrococcus meli- 

 tensis of the goat. 



At the same time, there are certain limitations imposed on 

 the demonstration of the present or previous existence of specific 

 antigens in the animal body through the examination of its serum 

 for specific antibodies, and these limitations are. in some cases, 

 due to the fact that the antibodies are present in too small a 

 quantity to be detected by our present methods, and. in others, 

 to the difficulty in obtaining a suitable preparation to act as the 

 antigen necessary for the performance of the tests. 



The sero-diagnostic methods which have found the most 

 common usage in veterinary medicine up to the present time, are 



