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 protozoal diseases the 

 micro-organisms occur for the greater part as intracellular parasites. One 

 attack of the disease does not as a rule confer immunity, and the treatment 

 has to be sought along the lines of medication 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 incuba- 

 tion 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 characterised 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 diphtheria and tetanus 

 bacilli are examples, secrete in the surrounding culture-fluid substances 

 which act as virulent poisons when injected into animals. Other bacteria 

 do not form such extracellular toxins, but in their case it is found that, if 

 the bodies of the bacilli be broken up, the injection of the contents of the 

 bacteria is attended with poisonous effects. The bacteria may be thus 



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