PLASMA 



165 



logical importance, for otherwise we might bleed to death even from 

 a small wound. 



Blood Plates. — A substance called throm'hin is the active agent 

 in changing fibrinogen to the insoluble fibrin of a clot. This 

 change seems to be due to the action of minute bodies in the blood 

 known as hlood plates. Under abnormal conditions these blood 

 plates break down, releasing some substances which eventually (if 

 the blood has the necessary content of calcium) cause the thrombin 

 to do its work. 



Disease-resisting Functions of the Plasma. — It is common 

 knowledge that some of us '* take " catching or communicable dis- 

 eases more easily than others. Some 

 fortunate persons are immune to certain 

 diseases, that is, they do not take them, 

 because certain antibodies are present in 

 their blood. These antibodies act in dif- 

 ferent ways, but their work is directed 

 against bacteria which get into the body 

 and cause disease. Some antibodies, 

 called ly'sins, have the power to dissolve 

 cells. Others, called aggWtinins, cause 

 the bacteria in the blood to become stuck typhoTd germs dump together 



together in little inactive masses, so that when added to the serum of a 

 .^ r J.1 1 X person who has typhoid. 



they are an easy prey tor the phagocytes. 



We have aheady heard of the work of the opsonins, another 

 kind of antibody. Agglutinins and other kinds of antibodies 

 called precip^itins have become a great help to physicians in de- 

 termining whether a person has a given disease. For example, 

 a test known as the Widal (ve-daF) test is now used in all hos- 

 pitals to determine if a person has typhoid fever. A few drops 

 of blood from the patient is allowed to stand until the serum 

 has separated, this is then diluted with a weak salt solution and 

 to this are added some living typhoid bacteria. If the person 

 has typhoid, the bacteria added to his serum will immediately 

 become agglutinated, thus showing that his antibodies are already 

 formed and at work. This is only one of a number of tests that 

 have been developed in recent years. Just as each disease is 

 caused by a specific kind of organism, producing a specific kind 



H. NEW CIV. BIOL. — 12 



In the Widal test, free 

 swimming, widely separated 



