626 MAN AS A CONQUEROR 



immunity to diphtheria, while the North American Indian is some- 

 what immune to scarlet fever. The natives of South America are 

 much more resistant to malaria and yellow fever than are whites from 

 more northern territories. Evidently, then, immunity may be racial 

 as well as individual. 



Immunity is also brought about through an attack of infectious 

 diseases. One Greek historian who visited Athens more than twenty 

 centuries ago noted that those who recovered from a visitation of 

 plague did not take the disease a second time. Immunity which 

 lasts for a greater or lesser period is usually found after attacks 

 of smallpox, chicken-pox, measles, mumps, scarlet fever, whooping 

 cough, and many other diseases. 



The Mechanism of Immunity 



All toxins, when entering the human body, cause the body cells 

 and blood to react to these poisons, through the protection of various 

 substances known as antibodies. These, when produced in the body, 

 have the effect of either neutralizing the toxins or actively fighting 

 bacteria. In addition to antibodies there is also a protective mecha- 

 nism (phagocytes) in the white corpuscles of the blood. If bacteria get 

 into a wound, for example, the phagocytes are apparently drawn to 

 the spot, possibly through some chemical stimulus, and attack the 

 bacteria by engulfing them. The blood contains certain types of 

 antibodies which are known as opsonins. These, which are specific for 

 different diseases, enable the phagocytes to engulf and digest invad- 

 ing bacteria. 



Certain other antibodies called lysins act directly on the bacteria 

 themselves, causing them to dissolve. Still another group of anti- 

 bodies called agglutinins cause the bacteria in the blood to clump 

 together in tiny inactive masses and are doubtless acted upon by 

 both opsonins and lysins so that they become an easy prey for the 

 phagocytes. Yet another group of antibodies, known as precipitins, 

 cause the bacteria to precipitate out from the blood in masses that 

 are easily discernible under the microscope. Agglutinins and precipi- 

 tins have become of great value to physicians in determining whether 

 or not certain diseases are present. For example, a test known as the 

 Widal test has been developed to determine whether a person has 

 typhoid fever. A few drops of the patient's blood are allowed to 

 stand until the serum has separated, and this is then diluted with a 

 weak salt solution to which are added live typhoid bacteria. If the 



