48 BACTERIOLOGY 



e.g. common colds, sore throats, bronchitis, gleet, cystitis, 

 etc., though in some conditions the results have not come up 

 to expectation. The method is really that of stimulating the 

 defensive powers of the patient and teaching his tissues to 

 help and defend themselves. The various methods of treating 

 tuberculosis by the tuberculins is of this nature, and, thougn 

 to some extent it is not yet beyond the experimental stage, 

 something may still be hoped from it in the future. 



Another important way of conferring immunity, quite dif- 

 ferent from the above in its mechanism, is known as passive 

 immunity. In this form, the substances or antibodies 

 already produced by another animal are withdrawn, usually 

 in its blood-serum, and then injected into the animal or patient 

 to be protected against the same disease as that against which 

 the first animal has been actively immunised. This method is 

 sometimes employed for obtaining immunity against bacteria 

 themselves, and sometimes against their toxic products only. 

 These are known respectively as antibacterial and antitoxic 

 sera. As examples of the former of these, anti-streptoccccal 

 and anti-plague sera, and of the latter, anti-diphtheritic and 

 anti-tetanic sera may be mentioned, and it will be obvious that 

 the chief use and value of these anti-sera will be for the treat- 

 ment of diseases already present. Antitoxic sera aid by 

 neutralising the toxins of the bacteria ; anti-bacterial sera 

 attack the bacteria themselves, both thus protecting the tissues 

 of the patient and giving him a better chance of fighting the 

 disease. Their action is more or less transitory, and repeated 

 injections of the serum may be required. Protective inocula- 

 tion by anti-sera is also useful where persons have been or 

 are likely to be exposed to infection, or are suspected to be in 

 the incubation or preliminary latent period of a disease such 

 as diphtheria or tetanus. 



A promising new line of treatment now being tried is the 

 combination of a vaccine and its corresponding anti-serum 

 inoculated at one time into the patient to be protected. 



CHAPTER VI 



MODES OF THE ENTRANCE OF PATHOGENIC BACTERIA 

 INTO THE BODY, AND THEIR ACTION 



How can we determine that a certain organism is the cause of 

 a given disease ? At the beginning of the " bacteriological 

 era," when pathogenic bacteria were first isolated and identi- 

 fied, Eobert Koch set forth certain postulates, usually known 



