GENERAL ZOOLOGY 



material gives only transitory symptoms of the disease. Thereafter, these 

 antibodies are responsible for an artificial acquired immunity to the disease in 

 question; this immunity may be permanent or may have to be renewed at 

 intervals. Dead bacteria are used for inoculation against typhoid and para- 

 typhoid; viruses of reduced virulence, called attenuated viruses, are used in 

 vaccination against smallpox or inoculation against rabies. The toxins, or 

 poisonous substances, produced by the bacteria are injected in inoculation 

 against diphtheria. 



Antibodies produced in other animals can be used to give passive immunity, 

 usually of short duration, or to combat antigens in human beings in the 

 control of disease. Thus, antibodies, also called antitoxins, against the toxin 

 giving symptoms of diphtheria are produced by horses inoculated with the 

 toxins of the diphtheria bacilli and can be obtained in the serum which 

 separates when the horse's blood clots after being drawn. Such immune serum 

 administered to an individual enables him to combat effectively the toxin or 

 poison of the diphtheria bacillus before antibodies are produced in adequate 

 quantities in his own tissues. After a wound is received under circumstances 

 where tetanus bacilli might be present, tetanus antitoxin, also from horses, is 

 routinely administered, in order to protect against the toxins these bacilli 

 would produce. Soldiers in World War II were given inoculations of tetanus 

 toxin and antitoxin in order that they might acquire artificial immunity 

 against this rapidly fatal bacterium. 



It may be noted here that some individuals have what is called natural 

 immunity to certain diseases. This immunity may be the result of inheritance 

 of a capacity to form certain antibodies, such as the natural antibodies 

 against antigens A and B of the red .blood cells. Very slight and unnoticed 

 infection with the causal agent of the disease may also bring about what seems 

 to be natural immunity. 



Immunity and resistance to disease must not be confused. Immunity is 

 protection against a specific pathogenic agent. Resistance is non-specific 

 and may depend on many factors. 



The quantity of blood in the circulatory system is important in connection 

 with the ease of its circulation and the adequacy of supply to all regions. 

 When blood is lost in amounts insufficient to cause death from oxygen want, 

 a condition known as shock may result. This condition can be controlled 

 by increasing the blood volume through the addition of plasma without the 

 red cells. Typing is not necessary for such transfusions. In World War II 

 one of the greatest contributions of science to the saving of human life was 

 the development of methods of separating the plasma from the great quan- 

 tities of blood donated by non-combatants, preventing it from clotting, and 

 drying it in such a way that it would keep indefinitely. The dried powder 

 was sent to all fronts along with triple-distilled water, with which it was 

 mixed before use in combating shock in wounded men. 



Lymph differs from blood in that it does not contain erythrocytes and 

 granular leucocytes. The plasma of lymph is derived from the blood by 



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