PRACTICAL APPLICATIONS 275 



>*! 



and yet the animal enjoyan immunity. Bacteria that have been acted 

 upon by an agglutinin are apparently not altered in appearance, viabil- 

 ity, or virulence. 



Many observations tend to show that the agglutinating power of a 

 serum gives no indication of the degree of immunity that exists. For 

 instance, relapses may occur in typhoid fever at a time when the agglu- 

 tinating power of the patient's blood is at its highest. 



At present agglutinins are regarded as playing a subsidiary role in 

 immunity, their presence being of diagnostic value, and an indication of 

 the presence of more important factors, and as an aid to bacteriolysis, 

 and phagocytosis. 



PRACTICAL APPLICATIONS 



The agglutination reaction is used for the following purposes: 



1. For thp diagnosis of disease, by identifying the bacterial infection 

 from which the patient is suffering. To do this satisfactorily we must 

 have on hand stock cultures of bacteria, and test the patient's serum for 

 agglutinins for these bacteria. For instance, if a patient presents symp- 

 toms of typhoid fever, the serum is tested for typhoid agglutinins; if 

 the reaction is very weak or negative and continues so, the serum is 

 further tested for agglutinins for Bacillus paratyphosus A and B. 



In typhoid fever the Gruber-Widal reaction may be positive as early 

 as the third day; usually, however, the positive reaction is obtained 

 somewhat later about the seventh or the eighth day. A day or so 

 earlier the bacilli used in making the test may be seen to lose their mo- 

 tility, and two or three may form a loose clump. This is the doubtful 

 reaction, and it is well to test every day or every other day until a de- 

 cisive reaction is obtained. 



According to Park, " about 20 per cent, of typhoid infections give 

 positive reactions in the first week; about 60 per cent, in the second 

 week; about 80 per cent, in the third week; about 90 per cent, in the 

 fourth week, and about 75 per cent, in the second month of the disease." 

 In about 90 to 95 per cent, of cases in which repeated examinations are 

 made a positive reaction is to be found at some time during the patient's 

 illness. 



Occasionally the reaction appears first during the stage of convales- 

 cence, and at times it may even be absent, the diagnosis being confirmed 

 by cultivating typhoid bacilli from the blood. The possibility of a given 

 case reacting strongly one day and weakly or entirely negative a day or 

 so later has been emphasized elsewhere. 



