138 BIOLOGY FOR BEGINNERS 



in the blood by the presence of the bacteria themselves. Thus 

 the attack tends to produce a defence against itself, if the body 

 be healthy. This natural resistance to disease is called natural 

 immunity, and constantly protects us from germs of whose pres- 

 ence we are entirely unconscious. 



To provide conditions favorable to resist disease it is evident 

 then that general good health is essential, aided by cleanliness, 

 pure and abundant food, light, air, and whatever will keep each 

 cell of our body keyed up to repel the invader before his rapid in- 

 crease gives him the advantage. We know how often when the 

 body is " run down," diseases are contracted, which would other- 

 wise be fought off without our knowing that the bacteria had 

 attacked us. How often a " mere cold " develops into some serious 

 ailment, because the cold, though perhaps not regarded as serious, 

 lowers the resisting power of the body and then bacteria find en- 

 trance and overcome our physiological garrison. 



Defence by Antitoxins. In case the bacteria do find lodgment 

 in our bodies, there is usually a period of some days between the 

 time of exposure and the actual illness: this period of incubation 

 is the time in which the bacteria are overcoming the body's first 

 resistance and multiplying sufficiently to gain the advantage. 

 Then the colonies of bacteria develop in some organ, as when 

 diphtheria bacteria attack the throat. The throat is not the only 

 portion harmed, for the bacteria also secrete a poison (toxin) which 

 causes more serious trouble to other organs of the body. If the 

 patient recovers it is because his body has been able to gradually 

 increase the amount of antitoxin in his system and so overcome the 

 poisons produced by the bacteria which are causing the disease. 



White Corpuscles. The lymph glands in various parts of the 

 body produce white corpuscles, and if the body is in good con- 

 dition at the tune of disease attack, they greatly increase the num- 

 ber of these defenders. These corpuscles are able to actually " eat 

 up " the bacteria or else carry them back to the lymph glands 

 where they are destroyed. 



Opsonins are chemical substances in the blood whose function 

 is not thoroughly understood, but which have to do with com- 



