IMPORTANCE OF ANTIBODIES IN DIAGNOSIS 383 



that in approximately 100 per cent of tlie cases of typhoid fever, 

 it will be pasitivc at some time during the course of the disease. 



Agcg-lutinins in Carriers. — It should also be borne in mind that 

 clironic carriers or persons who have been recently vaccinated 

 against typhoid may have a sufficient amount of agglutinins in 

 their blood to give a positive Widal. If they should develop 

 influenza or a streptococcus endocarditis or miliary tuberculosis, 

 or any other infection that gives a clinical picture similar to 

 typhoid fever, the Widal might be positive and therefore lead to 

 a mistaken diagnosis. Thus it will be seen that there are many 

 l)0ssibilities of error not only in performing the test but also in 

 the interpretation of the results. 



To understand the significance of diagnostic tests based upon 

 the presence of specific agglutinins in the blood of patients, one 

 needs to know something of normal agglutinins and of the effect 

 of vaccination and of infection on agglutinin titer. 



Normal Agg-lutinins and Diagnostic Titers. — The lilood of 

 normal individuals not infrequently contains a small amount of 

 agglutinins for various organisms. The normal agglutination titer 

 for E. typhosa varied from to 1 :20 with an occasional finding 

 somewhat higher, but with the average being about 1 :5. Titers of 

 1 :80 are considered of diagnostic value. In the case of P. tular&nsis 

 or Br. abortus the normal agglutinating titer may be somewhat 

 higher than for E. tjiphosa. Titers should be about 1:80 to sug- 

 gest active infection with either of the organisms. In active infec- 

 tions one commonly finds a titer of at least 1 :320 or higher. The 

 occurrence of previous unrecognized infection is commonly men- 

 tioned as the explanation of the presence of normal agglutinins, but 

 there are a few individuals who believe that in some cases these 

 agglutinins may be normal physiological products just as the iso- 

 and hetero-hemagglutinins are. Perhaps both explanations are 

 valid. 



Flagellar, Somatic and Labile Agglutinins. — In 1903, Smith and 

 Reagh in their studies on a motile organism concluded that ag- 

 glutinins for the flagella as well as for the bacterial cell (soma) 

 were produced. This indicated that motile organisms possess 

 flagellar antigens as well as somatic antigens. Felix and Weil 

 (1917) although disagreeing with Smith and Reagh found two 

 antigens which they named "11" and "0" respectively. The 



