128 METHODS OF EXAMINING SERUM 



mixtures of equal parts of serum and of each of the dilutions of 

 culture are prepared and treated in the same way. The tubes 

 are then placed in the incubator for eighteen to twenty-four 

 hours at 37 C., and at the end of that time the contents of 

 each are tested as regards sterility by means of cultures. In 

 this way the greatest dilution in which the bacteria are com- 

 pletely killed off is ascertained. The number of bacteria in 

 the original culture per c.mm. can be counted by the method 

 given on p. 70, and thus the total number of bacteria killed 

 off by the quantity of serum used can readily be calculated. 



As will afterwards (see chapter on Immunity) be described in 

 greater detail, when an animal is immunised against a particular 

 bacterium the bactericidal action of its serum may be greatly 

 increased, and this depends on the development of a particular sub- 

 stance called an immune-body, which is comparatively thermo- 

 stable and is not destroyed at 55 C. To analyse the bactericidal 

 properties of such a serum, it should in the first place be heated 

 in order to destroy the normal complement. Then to each of a 

 series of sterile tubes we add (a) a quantity of normal unheated 

 serum insufficient of itself to destroy the bacteria, (b) a given 

 amount of the bacterial culture, and (c) varying amounts of the 

 heated immune-serum ]., '01, '001, etc. c.c. In this way we 

 can find the quantity of the immune-serum which gives the 

 maximum bactericidal action. 



In some cases, however, when an animal is immunised against 

 a given bacterium, or when a patient is infected with the 

 organism, the serum may not have increased bactericidal action, 

 but nevertheless contains an immune-body which leads to the 

 absorption or fixation of complement. In other w r ords, the 

 immune-body is a substance which, along with the corresponding 

 or homologous bacterium, binds complement (p. 130). In order, 

 however, to explain the methods by which the fixation of com- 

 plement may be demonstrated, we must first of all give some 

 facts with regard to hsemolytic sera. 



Methods of Haemolytic Tests. A hamiolytic serum is usually 

 prepared by injecting the red corpuscles of an animal into the 

 peritoneum of an animal of different species the corpuscles of 

 the ox are most frequently used, and the rabbit is the most 

 suitable animal for injection. The corpuscles ought to be com- 

 pletely freed of serum by repeatedly washing them in sterile 

 salt solution, and centrifugalising. An injection of the corpuscles 

 of 5 c.c. of ox's blood, followed by two injections, each of 10 c.c., 

 at intervals of eight days, will usually give an active serum. The 

 animal should be killed by bleeding it, aseptically as far as poss- 



