THE CHEMICAL MECHANISMS OF DEFENCE 1159 



on which it acts and to which it becomes linked by its haptophore 

 group. 



CYTOLYSINS. The bacteria of tetanus and diphtheria cannot 

 exist in the body, infection by them being limited to a surface or abscess 

 cavity. When a disease involves infection of the tissues themselves by 

 living micro-organisms, somewhat more complicated mechanisms are 

 brought into play for the defence of the organism. We have already 

 seen that normal blood-serum may exert a paralytic or destructive 

 action on bacteria. Light has been thrown on the factors involved in 

 this destruction by a study of the phenomenon of hcemolysis, i.e. the 

 destruction of red blood-corpuscles. Normal goat's serum may be mixed 

 with the red blood-corpuscles of the sheep without any injury to the 

 latter. If, however, sheep's corpuscles, previously washed in normal 

 saline, be injected at intervals of a few days into a goat, the goat's serum 

 is found to have acquired the power of rapidly dissolving the red blood- 

 corpuscles. This haemolytic power is obvious, since it is only necessary 

 to mix the serum and the washed blood-corpuscles together and allow 

 the mixture to stand in a narrow tube. The corpuscles rapidly sink to 

 the bottom, leaving the colourless serum above, unless haemolysis has 

 occurred, in which case the serum will be of a transparent red colour. 

 If the haemolytic serum be heated to 55 C. it is found to have lost its 

 power of dissolving sheep's corpuscles. This power is at once restored 

 if to the heated serum be added any normal blood-serum, even of the 

 sheep itself. It seems therefore that two substances are involved 

 in the haemolysis, namely, (a) a substance present in most normal 

 sera which is destroyed at a temperature of 60 C. and has been called 

 the complement, and (6) a substance present in the serum only as a 

 result of the previous injection of some species of red blood- corpuscle, 

 which is resistant to the action of heat, and is called the amboceptor. 

 The reason for these names will be at once apparent from the following- 

 experiment. Haemolytic goat's serum is mixed with sheep's red blood- 

 corpuscles and the whole mixture kept at C., at which temperature 

 haemolysis is indefinitely delayed. After some time the corpuscles 

 are separated by means of the centrifuge. On testing the supernatant 

 fluid it is found to have no action on sheep's corpuscles, though it still 

 possesses the power of activating another specimen of serum which 

 has been heated. The serum separated from the corpuscles has thus 

 lost the amboceptor, but retained the complement. The amboceptor 

 is found to have attached itself to the red blood-corpuscles. If these 

 be washed and then added to normal sheep's serum, i.e. serum con- 

 taining the complement, they are rapidly dissolved. When solution 

 has taken place both complement and amboceptor are found to have 

 disappeared. The function of the amboceptor thus seems to be to 

 enable the complement already present in normal serum to act upon 



