476 MAN CONTROLS HIS ENVIRONMENT FOR HEALTH 



this or that antitoxin or anti-serum or vaccine, we must remember 

 those pioneers, Jenner and Pasteur, who took the first steps in 

 controlling germs, and began the work which may result finally 

 in preventing many diseases. 



How the body protects itself. We have already learned that 

 the blood contains small amounts of various protective sub- 

 stances, known collectively as antibodies. These help the cells of 

 the body combat harmful bacteria, the poisons or toxins which the 

 bacteria give out, and the poisonous " split proteins " which are 

 thrown into the blood when these bacteria die. When any protein 

 substance decays, or is only partly digested, it breaks down into 

 simpler substances. Some of these simpler proteins are poisonous 

 and are called ptomaines (to'ma-mz ; Gr. ptoma, dead body). 

 Ptomaine poisoning, while not so common as was once thought, 

 sometimes causes discomfort and even death. 



Practical Exercise 14. With the help of a physician, list all the diseases 

 for which immunity has been developed. 



Practical Exercise 15. What is immunity? Why are some persons more 

 likely to take a disease than others? Why do some people have a disease more 

 severely than others ? Why does travel bring increased likelihood of disease ? 



Self-Testing Exercise 

 Check the correct statements for your workbook : 



T. F. 1. A person is immune to a catching disease if, when ex- 

 posed to it, he does not take it. 



T. F. 2. Negroes are much more susceptible to measles and 

 tuberculosis than white people. 



T. F. 3. The resistance to a disease is largely determined by a 

 person's physical condition. 



T. F. 4. Protective substances, antibodies, in the blood help the body 

 to combat bacteria and their poisons. 



T. F. 5. Toxins are useful substances in the blood which help keep 

 us well. 



PROBLEM VI. WHAT ARE THE DIFFERENCES BETWEEN 

 ACTIVE AND PASSIVE IMMUNITY? 



Active and passive immunity. All toxins, when they enter the 

 human body, cause the body cells to react to the poison. If the 



