THE BLOOD AND LYMPH AS PROTECTIVE MECHANISMS 247 



At the end of three or four months a quantity of blood is withdrawn 

 from the jugular vein under antiseptic precautions, and is allowed to 

 clot. Its serum is then treated in a special way and is finally standard- 

 ized. Under favorable conditions, an animal yields from 250 to 800 

 units of antitoxin. For the prophylactic immunization of healthy 

 individuals about 500 units are required, while in a therapeutic way, 

 it may be administered in amounts ranging between 3000 and 20,000 

 units. 



Causes of Immunity. — In general it may be stated that the immu- 

 nity against microbic infection is dependent upon two processes, namely, 

 upon the phagocytic power of the leukocytes, in the presence of 

 opsonins (page 203), and secondly, upon the protecting influence of 

 certain substances, known as antibodies. As perfectly definite histo- 

 logical elements have not been discovered as yet in the fluids of our 

 body, these antibodies must be regarded as the products of precise 

 chemical correlations, i.e,, as agents that have been developed in the 

 course of physicochemical reactions between the cellular components 

 of the tissues and the invading unit, or antigen. Thus, the effective- 

 ness of an immunity must depend upon the quantity and quality of 

 the antibodies which are formed in consequence of the stimulating 

 action of the antigen. 



The place of origin of these bodies has not been definitely ascer- 

 tained. It is believed by some investigators that they are generated 

 somewhere in the body in the course of tissue metabolism, while others 

 hold that they are not independent elements but are derived from the 

 leukocytes. If the latter view is accepted, which is that of the French 

 school, the entire process of immunity constitutes merely a reaction 

 which is secondary to phagocytosis. The facts brought forth by 

 Ehrlich and his pupils, however, seem to contradict this conception.. 

 Thus, it has been found that the immune bodies exist preeminently 

 in the blood, but their presence can only be estabHshed in a physio- 

 logical way. In illustration of this statement it might be mentioned 

 that the destructive power of sera upon bacteria is lessened by allowing 

 them to stand, or by heating them to 56° C. It has also been observed 

 repeatedly that immunized blood-serum is richer in globulin than nor- 

 mal serum. 



Many physiological facts might be cited to prove that an animal 

 is in actual possession of these antibodies and that it is equipped with 

 them either at birth, or later on in the course of its life. It is also of 

 interest to note that the formation of these bodies may be brought 

 about not only with the aid of bacteria and their products, but also 

 with the help of a number of different toxins of animal and vegetable 

 origin. Ehrlich,^ for example, has shown that specific antitoxins may 

 be formed against the poisons of some of the higher plants. Similar 

 protective substances have been obtained by Calmette^ against the 



1 Deutsch. med. Wochenschr., 1891. 



2 Compt. rend., 1894. 



