194 BACTERICIDAL OR ANTI-BACTERIAL IMMUNITY. 



wbich these may be effective, the promise of serum therapy, so long limited to antitoxic 

 immunity, seems now to be more encouraging. 



It is possible that the reason why the serum of an animal immunized against a 

 given pathogenic micro-organism is not protective is that neither this serum nor the 

 body fluids of the individual into whom it is injected for protective ends contains suffi- 

 cient or suitable complements. 



We have seen in our review of hesmolysis that htemolytic serum heated to 56 C C. 

 loses its lytic power owing to the destruction of the very labile complements. We have 

 seen further that this power is restored by the addition of a littls fresh serum from a 

 normal animal ; that is, serum containing complement. Now it has been found that this 

 "reactivation" of the serum, as it is called, can often be brought about by the sera of 

 various animals. Thus, for example, the serum of the guinea-pig adapted to the ery- 

 throcytes of the rabbit is lytic for these cells of the rabbit. If such sera be heated to 

 56 C. it is no longer lytic, the activities of the complement are destroyed ; but the 

 serum can be reactivated by a little fresh serum not only from a normal rabbit, but 

 from the goat and the rat. The serum of many other animals, however, is ineffective 

 under these conditions. The reason for this, of course, in accordance with Ehrlich's 

 hypothesis, is that the complements of the reactivating sera have combining capacity 

 with the special amboceptors, and so can become effective, while in other sera, the link- 

 ing of the complement to the red cells through the amboceptors being impossible, there 

 can be no restoration of the lytic action. 



It is not difficult to secure immune substances (amboceptors) by the adaptation of 

 animals to various kinds of pathogenic bacteria. These may be formed in such abun- 

 dance as to be out of proportion to the complements. But unless these immune sub- 

 tances, when injected into the body for protective purposes, either carry with them or 

 find in the new environment an abundance and appropriate forms of complements, 

 they are not wholly available in destroying bacteria. One of the great problems of the 

 immediate future, then, so far as serum therapy is concerned, seems to be to secure 

 suitable complements to act with immune substances if the former do not exist in the 

 human fluids, or to reinforce these substances from the sera of suitable animals if the 

 human stock be scanty. There is, however, much ground for believing that in order 

 to be most effective the complements with which we may seek to reinforce the potency 

 of bacteriolytic sera in man should come from species closely allied to him. 



If the securing of an appropriate complement is thus of such importance in the 

 attempt to prepare bacteriolytic sera for therapeutic purposes, the maintenance of suffi- 

 cient complements in the human body must be of the utmost significance in its intrinsic 

 protective mechanism against infection. That this consideration is no-t without support 

 in fact is shown by the studies of Abbott, Longcope, and others, 1 who have found that 

 after the continuous administration of alcohol and in various chronic as well as acute 

 diseases, the amount of complement in the blood may be notably reduced. We have 

 thus a definite contribution to our knowledge of one of those factors in predisposition 

 to infection which, in a general way, are so fully recognized, but which are, for the 

 most part, but ill-defined and little understood. 



The Specific Character of Artificial Immunization. It is not yet possi- 

 ble to say in many cases to what extent the immunization effected in any 

 of the various ways indicated above is specific. In some cases it appears 

 to be so. That is to say, the protection which is afforded, for example, 

 by an attack of diphtheria or by the gradually increased administration 

 of the diphtheria toxin, or by the use of the immunizing serum, is limited 

 to this particular disease, and is not to be secured, at least in such marked 

 degree, by the use of other bacteria or bacterial products. In some in- 

 stances, on the other hand, immunization against one micro-organism or 



1 For a study of this subject see Longcope, Jour, of Hygiene, vol. iii., 1903, p. 28. 



