SECTION II 

 THE CHEMICAL MECHANISMS OF DEFENCE 



IMMUNITY. All infectious diseases are caused by the agency of 

 micro-organisms. The greater number of these, the bacteria, belong 

 to the class of fungi or schizomycetes ; a certain number must be classed 

 with the yeasts, while others are protozoal in character. It is especially 

 in the first class of diseases, namely, those due to bacteria, that the 

 organism has developed chemical mechanisms of defence. In the pro- 

 tozoal diseases the micro-organisms occur for the greater part as intra- 

 cellular parasites. One attack of the disease does not as a rule confer 

 immunity, and the treatment has to be sought along the lines of medica- 

 tion by drugs rather than by the development of methods of protection 

 normally displayed or developed by the animal which is the subject of 

 the infection. The diseases due to bacteria include diphtheria, tetanus, 

 tubercle, anthrax, pyaemia, and many others. In these diseases we 

 have to deal with a number of phenomena more or less common to 

 all. The infection in each case is due to the actual transference of 

 the specific organism from one animal to another. After the micro- 

 organism has attained entrance into the system there is a period of 

 incubation before the disease actually breaks out. When this occurs 

 the specific microbe is to be found in large quantities either in the 

 blood or in the tissues of the body. The disease is generally charac- 

 terised by fever and often by local lesions, such as the intestinal ulcers 

 of typhoid, or the glandular swellings of bubonic plague. The micro- 

 organisms may develop in the animal until its death, or the disease 

 may terminate in recovery and the total disappearance of the microbes 

 from the body. After recovery it is found that the patient is protected 

 from reinfection by the bacterium which was the cause of the disease, 

 and this condition of immunity may last as long as the patient lives. 

 The incidence of these bacterial diseases is not the same for all animals, 

 so that in the case of many diseases we can speak of a natural immunity 

 of certain animals for the diseases in question. 



The pathogenic micro-organisms can, in a number of cases, be culti- 

 vated on artificial media outside the body. It is then found that 

 they may be divided into two classes. One class, of which the diph- 

 theria and tetanus bacilli are examples secrete into the surrounding 



culture fluid substances which act as virulent poisons when injected 



1152 



