122 METHODS OF EXAMINING SERUM 



simple method, as follows : A piece of quill tubing is drawn out 

 to a capillary diameter so as to make a pipette about 6 inches 

 long. The point is broken off, and a rubber nipple adjusted to 

 the wide end ; a mark is made with an oil pencil about three- 

 quarters of an inch above the orifice. Blood is drawn from the 

 finger up to the mark, then an air-bubble is allowed to pass in. 

 A thin emulsion of the bacterium to be tested having been pre- 

 pared, a quantity of this is also drawn up to the mark. The 

 two fluids are thoroughly mixed by being first blown out on 

 to a sterile slide and then being drawn back into the pipette 

 and expelled, this being repeated several times. A cover-glass 

 is placed over the drop, and the slide is placed in the incubator 

 at 37 C. for fifteen minutes. The cover-glass is then slipped 

 off so as to make a film preparation, which in the case of 

 ordinary bacteria may be stained by Leishman's method. The 

 number of bacteria present in, say, fifty polymorphonuclear cells 

 successively examined is determined, and an average struck. The 

 method was first used for showing that in cases of staphylococcus 

 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. In making such an 

 observation, drops from the two mixtures are placed on the same 

 slide under separate cover-glasses, and the preparation incubated. 

 One cover is then slipped to one end of the slide, and the other 

 to the other, the two films being then stained as one. 



Leishman's method gives what may be called the total phago- 

 cytic capacity of the blood, but according to Wright's view the 

 process of phagocytosis in blood outside the body is not a 

 simple one, and before a leucocyte takes up a' bacterium the 

 latter must be acted on in some way by substances present in 

 the serum, which Wright calls opsonins (see Immunity). The 

 technique by which the actions of these opsonins is studied 

 has been elaborated by Wright and his co-workers in connec- 

 tion with bacterial vaccines, especially in relation to infection 

 by the pyogenic cocci and the tubercle bacillus. This technique 

 involves (1) the preparation of the bacterial emulsion, (2) the pre- 

 paration of the leucocytes, (3) the preparation of samples of (a) 

 serum from a normal person, and (b) serum from the infected 

 person. 



(1) Preparation of Bacterial Emulsion. In the case of the 

 pyogenic cocci, a little of a twenty-four hour living culture off 

 a sloped agar tube is taken and rubbed up in a watch-glass with 

 85 per cent, saline. The mixture is placed in a tube and centri- 

 fugalised, so as to deposit any masses of bacteria which may be 



