METHODS OF ELEMOLYTIC TESTS 129 



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. 72, 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 words, the 

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

 or homologous bacterium, binds complement (p. 131). 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 Hsemolytic Tests. A haemolytic 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- 

 ible, seven to ten days after the last injection ; the serum which 

 separates may be collected in suitable lengths of quill glass- 

 tubing drawn out at the ends, which are afterwards sealed in 

 the flame. To ensure sterility when the serum is to be kept 

 some time, it is advisable to heat it for an hour at 55 C. on 

 three successive days ; we have always found that serum treated 



