IMMUNITY 41 



and others. Wassermann has made a very efficacious anti- 

 meningococcus serum. 



Opsonins. Opsonins are substances in the blood-serum 

 which act on bacteria and prepare them for phagocytosis. 

 Opsonins can be increased by whatever increases immunity. 

 An increase is coincident with increased immunity. The 

 most common method of bringing about an increase is by the 

 injection of killed cultures of bacteria. 



Opsonins normally present in the serum are not specific. 

 Opsonins resulting from reaction to infection or inoculation 

 are specific. 



The opsonic index is the ratio between the number of 

 bacteria ingested by living leukocytes when operating in the 

 serum of a test and in normal serum respectively. 



After the injection of bacteria the opsonic index falls for 

 a short time. This period is called the negative phase, and 

 is followed by a rise in the index the positive phase. 



The " estimation of the opsonic index is a very complicated 

 way of finding out very little," and has been abandoned by 

 the great majority of workers. 



Antigens. Any substance that has, when injected into 

 the body, power to produce an antitoxin or antibacterial 

 body is called an antigen. 



The toxin of diphtheria, if injected, stimulates the normal 

 cells to produce chemic substances (free receptors) which 

 are at liberty to attach themselves to the active toxin mole- 

 cules and thus save the body cells from being acted upon; 

 toxin is, therefore, an antigen. 



Substances which have the power of destroying bacteria 

 are called bactericides; those which dissolve them merely are 

 called bacteriolysins. 



Hemolysis. When the hemoglobin of the red blood-cells 

 is liberated, hemolysis is said to occur. This is brought about 

 by the injection of certain substances, or hemolysins; these 

 are present normally in some sera, and can be developed in 

 others. Lysins and bactericidal substances seem to have 

 two parts one destroyed by heat (thermolabile), called 



