300 AGEICULTUEAli AND INDTJSTKIAL BACTBEIOLOGT 



with that of those engulfed when using normal serum, is 

 termed the opsonic index. For example, if patient serum 

 on the average should cause the white blood cells to engulf 

 an average of ten bacteria, and the normal serum cause the 

 white blood cells to engulf five bacteria, the opsonic index 

 would be the ratio of ten to five. 



The Presence of Opsonins in Antisera.— Certain types 

 of antisera are prepared by injection of suitable animals 

 with dead or living cultures of bacteria in an effort to secure 

 a high concentration of opsonin specific for the disease in 

 the blood of such an animal. Such sera are sometimes used 

 in the treatment of disease. Most of the so-called anti- 

 bacterial (not antitoxic) sera used in disease treatment con- 

 tain opsonins and usually cytolysins as well. 



Significance of Opsonins in Immunity. — It is probable 

 that the body owes more of its resistance to disease to the 

 presence of opsonin than to the presence of any other single 

 antibody. Development of opsonin may be stimulated by 

 the use of vaccines. A vaccine may be defined as a dead or 

 attenuated culture of an organism injected into an animal 

 or individual for the purpose of causing it to develop an 

 active immunity. The immune substances formed are prob- 

 ably in large part cytolysin and opsonin. In some cases 

 the term vaccme is restricted to living or attenuated bac- 

 teria, and for dead cultures injected in a similar fashion the 

 word bacterin has been coined. 



It is possible, following vaccination, to note increased 

 resistance by studying the opsonic index of the vaccinated 

 individual. 



Htpersusceptibilitt or Anaphylaxis 



In certain respects the phenomenon of anaphylaxis or 

 hypersusceptibility is the converse of immunity. It is, 

 nevertheless, very closely related to the latter phenomenon 

 and should be discussed, in consequence, at this point. 



