198 General Biology 



familial immunity, and if only an individual is intensely resistant to a 

 given disease or injury, we speak of it as individual immunity. 



Immunity is also divided into (1) natural immunity (under which 

 racial immunity can be classified), and (2) artificial immunity. The 

 second of these divisions is again subdivided into active and passive 

 immunity. 



Bacteria either contain poisonous substances (endotoxines), as does 

 the typhoid fever bacillus, or they produce poisonous substances (merely 

 called toxins or ectotoxines) as does the diphtheria bacillus, tetanus 

 bacillus, etc. 



Now, if these toxic substances are distributed within the body, the 

 body tries to protect itself by manufacturing antitoxins, which are 

 opposing substances for the purpose of neutralizing the poisons and thus 

 preventing them from injuring the system. If the toxic poisons are not 

 too severe, the antitoxin prevents a disease from forming. 



As an example we may cite typhoid fever. Here the antitoxin is 

 not only manufactured, but actually remains in the body of the patient 

 for some years after the disease has passed away. The great quantity 

 of antitoxin present can, during these years, thus prevent another attack 

 of the disease if the typhoid bacillus again gets into the body. Such 

 an individual is, therefore, during the time the antitoxin is present in his 

 body, immune to new attacks of typhoid fever. He is immune because 

 his body produced immunizing bodies which protect him. His body is 

 active in producing these immunizing bodies, and such immunity is 

 therefore called an active immunity. 



An animal which, in a specific disease, also builds up such anti- 

 bodies or immunizing bodies, may have these antibodies removed from 

 its blood. The liquid part of the blood, which contains the antibodies, 

 is called an antibacteriological or antitoxic serum. Such sera are then 

 injected into human beings. As the person, into whom they are injected, 

 then has protective substances which he does not otherwise have, he 

 (without having his own body manufacture the antibodies) becomes 

 immune for a certain length of time to the specific disease for which the 

 antibodies were manufactured in the animal's body. Such an immunity 

 is. therefore, called a passive immunity. 



It is upon principles evolved from these facts that the various 

 vaccines have been brought forth against cholera, typhoid fever, small- 

 pox, etc., as preventive measures, as well as the therapeutic sera injected 

 after the disease is present, as in meningitis, diphtheria, tetanus, etc. 



We know that various chemical substances have definite affinities 

 for each other, and, to explain the mechanism of immunity, it is assumed 

 that every cell in the body has some particular chemical attachment or 

 affinity. Let us say that a certain molecule connected with each cell 

 has an affinity for various substances which pass through the body. 

 This molecule is called a receptor. We know that normally, as blood 

 passes the different cells of the body, the cells have a selective action; 



