Another type of chemical response to foreign substances is revealed by 

 the serum of a typhoid-fever patient (see illustration, p. 241). The new sub- 

 stance is called an agglutinin because it clumps the bacteria together in 

 masses. Like precipitins and antitoxins, agglutinins are specific; that is, 

 each acts only on a particular species of bacteria. The agglutinins do not 

 kill the bacteria, but probably interfere in some way with their action. It 

 is certain that in their presence the phagocytes more readily attack the 

 bacteria (see page 188). 



In the blood of a backboned animal, red and white corpuscles float about 

 unaffected by one another. But if blood from a different species is injected 

 into the veins of a rabbit or mouse, say, the foreign red corpuscles are pres- 

 ently destroyed. After the foreign cells are introduced, the body seems to 

 form a new substance that dissolves the invading material. Such specific 

 cytolysins, or "cell-dissolvers", are formed in response to various kinds of 

 cells or tissues and to various bacteria. Thus the serum of a rabbit that has 

 been treated with human blood will dissolve human corpuscles, but not 

 those of a goat or a monkey. 



Specific Tests of Disease^ The antibodies that develop after an infec- 

 tion or after an inoculation are specific and are present in the blood. They 

 therefore appear in the serum. We sometimes speak of such a serum as an 

 "immune" or as a "specific" serum. Because of the specific characteristics 

 of such altered serums, we can use them for the quick and reliable diag- 

 nosis of certain diseases, as the Wassermann test for syphilis and the Widal 

 lest for typhoid fever (see illustration, p. 241). Other tests tell us whether 

 a person is susceptible to a given disease or sensitive to a particular sub- 

 stance. The Schick test is used to show whether a person is immune or 

 susceptible to diphtheria. Similar tests are used to discover the plant or 

 animal substances to which sufferers from asthma or other "allergic" con- 

 ditions are sensitive. 



It has been possible to distinguish in the laboratory thirty-five or more 

 distinct types of pneumococcus bacteria that can cause pneumonia. It has 

 been possible to prepare specific serums for a few of these types. But physi- 

 cians are unable to recognize from the patient's symptoms which particular 

 type of germ is present ; and testing for type takes time, and sometimes every 

 hour counts. Before all the types could be readily distinguished, and before 

 dependable serums were available for more than a few types, biochemists 

 had found a more promising treatment. This is the use of the synthetic 

 drugs of the so-called "sulfa" series. These act alike on all types of pneu- 

 monia, as well as on gonorrhea and other diseases caused by bacteria of the 

 coccus group (see page 613). Individuals differ in their reaction to various 

 sulfa drugs, but research to improve these compounds is going forward 



iSee No. 3, p. 246. 

 242 



