THE BLOOD AS A LIVING TISSUE 199 



More than this, there are important specific differences. 

 Just as hens are quite indifferent to the action of morphin, 

 and as rabbits are insensitive to atropin, human beings are 

 quite immune to diseases that are serious or even fatal to 

 birds or cattle. Such immunity is called natural immunity, 

 and is inherited. In many cases it probably depends upon the 

 chemical peculiarity of the blood or the body juices ; in others 

 it depends upon the quick response of the live cells to the 

 poisons and other products of the bacteria. But such natural 

 immunity is not absolute ; that is to say, it may be weakened 

 or destroyed by various conditions. 



We may see from these considerations how important it is 

 to guard the natural immunity of the body against the destruc- 

 tive effects of undue exposure to extremes of temperature, to 

 excessive fatigue (or insufficient rest and sleep) or overwork, 

 and to prolonged hunger or thirst (or improper nutrition). We 

 can also understand the importance of suitable conditions and 

 habits of ventilation and of exercise in maintaining the resist- 

 ance of the body, and the danger of using drugs, alcohol, or 

 other substances that interfere with the action of the blood as 

 a living tissue. 



Not only the conditions that commonly affect the energy 

 and resistance of the protoplasm, but disease itself may affect 

 the resistance of the body. After pneumonia, for example, 

 one is more liable to catch other diseases than he is ordinarily, 

 and he is more liable to catch pneumonia after typhoid or 

 measles than he is ordinarily. One cannot afford to be sick 

 even a little ; that opens the way for more serious trouble. 



230. Acquired immunity. It is a well-known fact that one 

 who recovers from certain diseases is practically immune ; as 

 the common saying goes, "You can't have measles twice." 

 This is true of mumps, whooping cough, scarlet fever, typhoid 

 fever, and smallpox. This acquired immunity is no doubt 

 due to the substances produced in the blood in the course of 

 the disease. In the case of diphtheria and of some other 



