120 METHODS OF EXAMINING SERUM 



of those which remain. In practice, the method consists in 

 adding to the serum an equal volume of a thick emulsion 

 of the bacterium (the organisms being scraped off an agar 

 slope), allowing the mixture to stand at 37° C. for two or three 

 hours, and then separating the bacteria with the centrifuge. The 

 supernatant clear fluid is now pipetted off, and its agglutinating 

 properties studied on the other members of the bacterial group 

 either by sedimentation or by the microscopic method. The 

 object of the method is to determine which member of a 

 bacterial group is causally related to the condition from which 

 the serum is obtained, and examples of its application for this 

 purpose will be found in the chapter on Typhoid Fever (p. 393). 

 Here the principle is that, when an unknown strain belonging 

 to such a bacterial group is under investigation, if its capacities 

 for absorbing agglutinins from a serum are the same as those of 

 an already recognised strain, then the two are probably identical. 

 On the other hand, an allied strain to the organism by which 

 the agglutinin has been produced will absorb only part of the 

 agglutinin. 



Opsonic Methods. 



Method of measuring the Phagocytic Capacity of the 

 Leucocytes. — This was first done by Leishman by a very 

 simple method, as follows : — 



Equal quantities of blood and of a fine emulsion of the bacterium to be 

 tested are mixed together, a small drop of the mixture is placed on a 

 glass slide and covered with a cover-glass ; the preparation is placed in 

 the incubator at 37° C. for fifteen minutes. The cover-glass is then 

 slipped off and the film on the slide stained by Leishman's method. A 

 control preparation can be made with normal blood in the same way and 

 the two films are stained as one. The number of bacteria present in, say, 

 fifty polymorphonuclear cells successively examined is determined, and an 

 average struck. 



By this method Leishman showed that in cases of staphylo- 

 coccus infection the average number of bacteria taken up was 

 less than in a control in which the same bacterial emulsion was 

 exposed to the blood of a healthy individual. Wright subse- 

 quently showed that phagocytosis depended upon certain sub- 

 stances in the serum to which he gave the name opsonins {see 

 Immunity) and elaborated a method by which its degree could 

 be estimated. The technique involves (1) the preparation of the 

 bacterial emulsion, (2) the preparation of the leucocytes, (3) the 

 preparation of samples of (a.) serum from a normal person, and 

 (b) serum from the infected'person. 



