386 IMMUNOLOGY 



With typhoid fever in an unvaeeinated individvial the agghitinin 

 titer does not increase perceptibly nntil the fifth or seventh day 

 of the disease. Occasionally it is the third week before an increase 

 can be noted. The titer occasionally may not go above 1 :100, 

 bnt as a rule it will go mnch higher. Hence early in the disease 

 when the ))acteria arc in tlie blood stream, a blood culture is the 

 lal)07*atoi'y procediii-c of choice since the Widal is likely to be 

 negative. During hiter weeks the opposite will hold true. 



The Usk of the \Vn)AL ix Vaccinated Ixdividi^vls. — Where an 

 examination is being made of tlie blood of an individual who has 

 previously been vaccinated against E. tjfphosa the question arises 

 as to whetlier the abnormally high titer is due to the vaccine or to 

 an actual existing infection. It is not impossible for a vaccinated 

 person to acquire an infection from massive doses such as may be 

 present in contaminated milk. When such a question arises, it 

 is answered by titrating the patient's blood daily using serial 

 dilutions of his serum, and noting whether there is an increase in 

 the titer, as one would expect if infection with E. typhosa is 

 present, or whether the titer remains constant as it might after it 

 had reached a Igav level several months following vaccination. 

 Since iiifections due to *S'. paratyphi A and <S^. paratyphi J?* 

 may resemble typhoid fever, it is customary to determine the 

 agglutinin titer for each of these wherever a routine Widal is 

 done. 



Importance of "Fine Granular" and "Loose Flocculation. " — 

 In interpreting a positive diagnostic agglutination reaction there 

 are those who believe that a "fine granular" type of agglutination 

 indicates a favorable outcome, while a large loose flocculation is 

 rather unfavorable. The present state of our knowledge relative 

 to the role of antibodies in resistance to infection has been dis- 

 cussed in earlier chapters. In agglutination work care must be 

 exercised to avoid overlooking fine granular agglutination. 



Time and Temperature of Incubation. — In regard to the time 

 and temperature of incubation, it should be remembered that while 

 incubation for several hours at room temperature or at 37° C. is 

 frequently satisfactory for the Widal, 50° to 55° C. is preferable. 

 The former conditions often give false negative reactions when one 

 is dealing with suspensions of Br. abortus or with P. tularensis and 

 for that matter with suspensions of the meningococcus used in 



*.S. schottmUUeri. 



