52 VACCINE AND SERUM THERAPY. 



importance in immunity and that the method of determining the 

 opsonic index is accurate; (b) that in the natural recovery from 

 an infection the opsonic index changes, as Wright aims to change 

 it, by the injection of bacterial vaccine; and (c) that the injection 

 of vaccine produces the changes described by Wright. 



IMPORTANCE OF OPSONIN IN IMMUNITY AND ACCURACY 



OF THE METHOD OF THE DETERMINATION 



OF THE OPSONIC INDEX. 



Spontaneous phagocytosis has been described from time to time. 

 With the introduction of Wright's opsonin theory and the develop- 

 ment of the technique of opsonic index determination, phagocytosis 

 has been found to occur, in most cases, only when fresh serum is 

 present. Neufeld and many others have found that contrary to 

 Wright's statement phagocytosis is not entirely dependent upon 

 opsonins, for bacteria with little virulence may be taken up by 

 the leucocytes even though no serum is present. The existence 

 of opsonin, however, and its relation to phagocytosis can no longer 

 be doubted; the part it plays in immunity has not yet been 

 satisfactorily demonstrated. Many of the objections made to 

 Metchnikoff's phagocytic theory of immunity present themselves 

 in the opsonic theory of immunity. It has been found that highly 

 virulent bacteria are not nearly as readily ingested by leucocytes 

 as are less virulent ones. Bacteria engulfed by leucocytes are 

 not always dissolved and disposed of for it has been demonstrated 

 by various investigators that certain species of bacteria produce 

 or secret substances that are antogonistic to and actually kill 

 leucocytes. Even if it is accepted that opsonin plays an important 

 part in immunity, the opsonic index determined by Wright's 

 method, need not necessarily be an index of the patient's im- 

 munity to certain infectious microorganisms. Agglutinins, bac- 

 teriolysins, and anti-toxins, all of which are substances better 

 known than are opsonins, are only circumstantial evidence of 

 greater or less importance in the complex phenomenon of immu- 

 nity. The body has various means of defense against bacterial 

 invasion and action so that the assumption that all the protec- 

 tive substances and means against bacterial diseases and infec- 

 tions manifest themselves in such a manner that they can be de- 

 termined from the opsonic index seems unreasonable. 



