VARIOUS ACTIONS OF SPECIFIC SERA. 487 



bacterium, and thus in nature to be allied with the process of 

 immunisation. 



Besides those stated above, other phenomena have been ob- 

 served in the interaction of anti-sera and the corresponding 

 bacteria. For example, it has been shown that when certain 

 bacteria e.g. the typhoid bacillus, B. coli, and B. proteus 

 are grown in bouillon containing a small proportion of the 

 homologous serum, their morphological characters may be 

 altered, growth taking place in the form of threads or chains 

 which are not observed in ordinary conditions. In other in- 

 stances a serum may inhibit some of the vital functions of the 

 corresponding bacterium. 



Summary with regard to Anti-sera. In a former chapter it 

 has been shown that in the production of disease by bacteria 

 there are two main factors concerned, viz., the multiplication of 

 the living organisms in the tissues and the production by them 

 of toxins. The facts which have been stated above show that in 

 the blood serum of highly immunised animals there are present 

 substances of remarkable potency which may act against either 

 of these two factors. In the first place, a serum may protect 

 against the separated toxin, or, in other words, may be antitoxic. 

 In the second place, a serum may lead to the destruction of 

 the organisms ; the term antibacterial is, therefore, conveniently 

 applied to such a serum. In many instances an antibacterial 

 serum has little or no effect against the toxins. The action of 

 both varieties of anti-sera is, within certain limits, specific, being 

 exerted only against the particular organism or toxin which has 

 been used in its preparation, having a definite value which can 

 be ascertained by experiment. It does not follow from what has 

 been said that a serum may not act in both of the ways described. 

 A given serum might, for example, be powerfully antibacterial 

 and feebly antitoxic at the same time. 



It is specially to be noted that anti-sera are not peculiar to the 

 case of bacteria and their toxins, but constitute a large group, 

 the characteristic features of which, in general terms, are, that 

 they are produced by the injection of complicated organic 

 substances, either in solution or as formed elements. This group 

 may be conveniently divided into two great classes. In one of 

 these the characteristic effect is apparently due to one substance 

 acting alone, whilst in the other this effect requires for its com- 



