AGGLUTINATION REACTION 231 



basis, thus securing constancy in the agglutinin unit. For 

 example, if a batch of standard culture proves to be twice as sensi- 

 tive to agglutination as the original standard, so that half the 

 amount of serum produces standard agglutination under test con- 

 ditions, the new standard culture is given a reduction factor of 

 double the size of the original factor i.e. 5. 



The Agglutination Reaction l 



The agglutination (Widal) reaction is principally employed for 

 the diagnosis of typhoid, paratyphoid and undulant fevers and 

 bacillary dysentery. For the recognition of bacterial species the 

 agglutination (Bordet-Durham) reaction is of wide application, 

 and its extension by the saturation test is still more delicate. 



The temperature should not be below 20 C., and agglutination 

 is more rapid at 37 C. For comparative observations, the 

 relative amount of culture should be the same in each specimen. 



Collection of blood. Blood may be collected from patients as 

 detailed at p. 141, and also in the section dealing with the Wasser- 

 mann reaction. 



If tubes are not available, the blood may be spotted on to a 

 piece of glass, cover-glass, or slide, glazed paper, tinfoil, etc., and 

 allowed to dry. For use, a drop of distilled water is placed on the 

 dry blood to dissolve it, and the solution used like serum. 



Dilution of the serum. This may be carried out in various ways, 

 with the haemocytometer pipette, with a simple pipette with 

 rubber teat as used for opsonin work (Fig. 36, a, p. 260), with a 

 throttled pipette, by the drop method (see pp. 58, 59) or even 

 with a platinum loop. With the simple pipette a little serum is 

 aspirated up so as to occupy 1^2 cm. of the stem, and the upper 

 limit is marked with a grease pencil or ink. A bubble of air 

 is next admitted, and then salt solution is aspirated up to the 

 mark, another bubble of air is admitted, and the process is 

 repeated again and again ; so that, finally, the pipette contains 

 1 volume of serum and 4-14 volumes of salt solution, each volume 

 being separated from the next one by an air-bubble. The contents 

 of the pipette are then expelled into a watch-glass and thoroughly 

 mixed, and further dilution of this dilution is performed in the 

 same manner. Two or three dilutions are usually made e.g. 



1 See Medical Research Council, Special Report Series, No. 51. 



