IMMUNOLOGY. IMMUNITY AND IMMUNIZING AGENTS 249 



marily as scavengers, feeding upon the multitude of bacteria present. 

 However, that may be, the amebic infection does induce very serious 

 disturbances which were difficult to cure until it was found that-ipecac 

 (emetin) was quickly fatal to the amebas of dysentery as well as to those 

 of pyorrhea (enteric pills, Lloyd alcresta ipecac tablets; or hypodermic 

 injections of the active principle of ipecac, emetin). 



The amebas found with decaying fruits and other vegetable matter 

 feed upon bacteria and yeast cells. An ameba found in decaying bananas 

 limited its diet almost entirely to yeast cells which it disintegrated very 

 quickly. 



In the ordinary conditon of phagocytosis as explained under staphy- 

 lococcic infection, with the resultant contra-invasion by the white blood 

 corpuscles, there is unquestionable an active destruction of objectionable 

 invaders (the bacteria), brought about by the special body protectors or 

 guardians, the leucocytes, through the assistance of the opsonins, and such 

 a condition may be designated patrocytosis as contrasted with the condi- 

 tion as we find it in amebic dysentery and in pyorrhea where the relation- 

 ship of ameba to the organism invaded (the human body) is harmful, even 

 though the amebas feed upon body bacteria. In malaria for example, we 

 have true parasitism. The invading organism (the malarial plasmodium) 

 does not feed upon or destroy body bacteria and does not render even 

 the slightest service to the human body. 



b. Opsonins. Opsonins have a direct relationship to leucocytosis or 

 phagocytosis (patrocytosis). Wright and others proved that there exists 

 in the serum of the blood and also in the cells of the body, substances which 

 possessed the property of so acting upon and modifying bacteria as to 

 render them more readily seized upon and devoured by the leucocytes. 

 Nothing is known as to the composition of these substances. No one has 

 succeeded in isolating them in a, pure state. They are unquestionably 

 proteid in nature combined with the serum and cell proteids of the body. 

 They are specific in their action. That is an opsonin will not act upon 

 different kinds of bacteria. The opsonin which acts upon the Staphylo- 

 coccus aureus so preparing these organisms as to make them more readily 

 seized upon and digested by the leucocytes, will not act upon or prepare 

 the organism which is causative of pneumonia, or any other species or 

 variety of pathogenic organism. Each species, and probably also each 

 variety of pathogenic microbe is acted upon by a special or specific 

 opsonin. 



The proof of the existence of opsonins is as follows: If fresh blood 

 is mixed with an emulsion of bacteria and incubated at body temperature 

 for an hour, it will be found that bacteria are within the white blood cor- 

 puscles. If we wash the blood corpuscles free from serum (by means 



