SPECIAL PROPERTIES OF BLOOD SERUM. 265 



small invading organisms they seem to accomplish by a kind 

 of digestive process which in a general way may be followed 

 under the microscope. To some extent the destruction of one 

 kind of blood by another may possibly be accounted for in this 

 way. But the chief action is certainly of a different character. 

 "While the white cells are in a measure protective agents, active 

 in destroying elements which would be harmful if left in the 

 blood, it seems altogether likely that the most important con- 

 serving forces in the blood are soluble compounds, possibly of 

 the nature of enzymes. This view has been gradually devel- 

 oped and rests on a basis of experiment and observation. 



Harmful foreign bodies entering the blood may be in the 

 nature of cells, as of bacteria, or they may be the poisons called 

 toxins produced by bacteria. Anything in the blood which 

 resists or overcomes the force of this invasion is called an anti 

 body. Normal serum seems to contain a number of anti sub- 

 stances, which have received different names, depending on 

 how or against what they act. Some are called precipitins, 

 others agglutinins, cytotoxins, etc., which terms will be ex- 

 plained later. In addition to the anti bodies normally present 

 in sera in variable amounts, and which confer a certain degree 

 of immunity, there may be produced artificially a greatly in- 

 creased specific immunity against some particular invasion. 

 It was the discovery of this fact which in reality led to the 

 systematic study of the whole phenomenon. 



The castor oil bean contains a peculiar poisonous principle 

 known as ricin, which if given in relatively large doses is fatal, 

 but against which an animal may be immunized by treatment 

 with gradually increasing small doses. An experiment made 

 by Ehrlich, to whom much is due in this field of investigation, 

 showed that the serum of the treated animal must contain now 

 a specific anti body capable of neutralizing the physiological 

 action of ricin. He found that if the ricin poison and the 

 serum of the immunized animal are mixed in vitro in certain 

 proportion, and then injected into a fresh non-immunized ani- 

 mal no toxic action follows. The serum of the first or im- 



