342 



BIOLOGY AND HUMAN LIFE 



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solid precipitate. The unknown substance in the serum is called a pre- 

 cipitin, and, like antitoxin, it is always specific: a precipitin formed 

 under the influence of a goat protein will react only to that protein. This 

 fact has been put to use in several ways : 



I. The precipitin test enables us to distinguish between a drop of blood 

 (or other material, like meat) from a horse, let us say, and a drop of 



blood from a dog or a human being. This is 

 often of great importance in legal trials. 



2. Experiments now going on make it likely 

 that this principle can be applied in the diag- 

 nosis of disease, enabling us to tell without 

 doubt which of several possible parasites are 

 present in a patient. 



259. Agglutinins. If some serum from the 

 blood of a typhoid-fever patient is mixed with 

 a few drops of liquid that contains living ty- 

 phoid bacilli, the bacteria will all be clumped 

 together in masses. There is apparently formed 

 in the blood (under the stimulus of some ty- 

 phoid protein) a substance that acts upon these 

 bacteria by sticking them together, or aggluti- 

 nating them. Such substances are called agglu- 

 tinins, and, like precipitins and antitoxins, they 

 are specific. The bacteria are not killed by the 

 agglutinin, although their free action is inter- 

 fered with (see Fig. i6i). 



260. Cytolysins. When we examine the 

 blood of an animal under the microscope, we 

 can see the corpuscles, red and white, move 



about unaffected by one another ; but if a little blood of one animal is 

 injected into the veins of another, the foreign red corpuscles are shortly 

 destroyed or dissolved by a specific substance. This cell-dissolving sub- 

 stance is not present in the blood all the time ; it is formed only after 

 the foreign cells are introduced. These cell dissolvers, or cytolysins, are 

 formed in response to any foreign cells or tissues, or to various bacteria, 

 and they are always specific. Thus, the serum of a person who has been 

 treated with dead typhoid cells will dissolve typhoid bacteria, but not 

 other species of bacteria. These facts are used practically in the fight 

 against typhoid fever. A measured quantity of dead typhoid germs is 

 injected into the body. The specific typhoid-cell dissolver, or cytolysin, 

 is formed by the action of the living cells of the body, especially the 



Fig. 1 6 1. Agglutination 

 ©f typhoid bacilli 



a, bacilli swimming about 

 separately; b, the same 

 clumped together, or ag- 

 glutinated. The Widal test 

 for typhoid fever consists 

 in mixing a few drops of 

 serum from the suspected 

 person with a quantity of 

 typhoid bacteria under the 

 microscope. If agglutina- 

 tion takes place, the per- 

 son is known to be infected 

 with typhoid germs 



