3i6 



Constajit Care Is Needed for Health unit y\ 



Another body defense. Pathogenic bac- 

 teria most often injure their host because 

 their protoplasm is poisonous to the host. 

 Sometimes the injury is the resuh of poi- 

 sonous substances that go out of the bac- 

 terial cell. These poisonous substances 

 made by bacteria are called toxins. Each 

 kind of bacterium produces a special 

 kind of toxin. We say that toxins are 

 specific. 



The presence of toxins in the body 

 causes it to produce substances which 

 make the toxins harmless. The substances 

 that make toxins harmless are called anti- 

 toxins. The body cells that produce anti- 

 toxins release them into the blood; the 

 antitoxins are thus carried throughout 

 the body. Antitoxins are also specific in 

 action, that is, an antitoxin neutralizes a 

 special toxin and has no eflfect on any 

 other toxin. We may compare it to a 

 key which fits only one lock. Thus diph- 

 theria antitoxin neutralizes only diph- 

 theria toxin; it does not affect tetanus 

 toxin. You will read more about anti- 

 toxin in the next problem. 



But antitoxins are by no means the 

 only protective substances in our blood. 

 We make opsonins, which are substances 

 that act on bacteria in such a way that 

 phagocytes can more easily devour them; 

 lysinSy which dissolve bacteria; and agglu- 



ti?ji?is, which cause bacteria to gather in 

 clumps. 



Immunity to disease. The antitoxins, 

 opsonins, lysins, and agglutinins are 

 called antibodies. Now do Exercise io. 

 In general, antibodies are made only 

 after the germ enters; they are not natu- 

 rally present in the blood as are the 

 phagocytes. Also, the antibodies are not 

 living cells. 



When an animal has produced anti- 

 bodies in combating a special bacterium 

 the animal is said to have acquired im- 

 immity to that disease. This immunity 

 may last a very short time, as in influenza, 

 or for many years, as in whooping 

 cough, scarlet fever, and generally in 

 smallpox. People in China and India, 

 four thousand years ago, deliberately ex- 

 posed their children to smallpox when- 

 ever there was a particularly mild epi- 

 demic, knowing that they would not 

 get smallpox a second time. See Exer- 

 cise 1 1 . 



Sometimes it happens that an individ- 

 ual or even a whole species has natural 

 inmninity to a disease. Apparently the 

 entering germ does not meet suitable 

 conditions in the host; the germ dies and 

 does no harm. For example, man has nat- 

 ural immunity to distemper, a deadly 

 disease of dogs. Now try Exercise 12. 



Questions 



1. Give a simple definition of disease and name four types of diseases. 



2. Define parasite and host. Define parasitism. What kinds of plants 

 and animals may be parasites? 



3. Describe bacteria as to size, structure, and shape. What activities do 

 they carry on? 



4. To raise bacteria what food and temperature conditions are necessary? 

 What does a bacteriologist mean by sterilizing? How does he do it? 



