208 OPSONINS 



These facts show that the agent which develops and stimulates 

 phagocytosis is in solution in the blood of the immunized animal, is 

 not present in the blood of the non-immunized animal, and is apart 

 from the leukocytes. 



The work of Denys and Leclef seems to have been ignored for 

 some years, and quite ignorant of it, Leishman in 1902 showed that 

 phagocytic action can be demonstrated in a drop of human blood 

 freshly taken. He determined the degree of phagocytic action by 

 counting the included bacteria in a number of phagocytes and showed 

 that these were greater in the blood of one vaccinated with staphylo- 

 cocci than in normal man. In this way, Leishman opened the way for 

 "vaccine therapy," which has since been widely employed. Wright and 

 Douglas developed this treatment and proposed that the substances in 

 the blood which favor phagocytic activity should be designated 

 "opsonins" (from the Greek, t<t><rwvfa f meaning, "I prepare the food"). 

 They showed that the opsonins in normal blood vary in narrow limits 

 and that they are greatly increased by the subcutaneous injection of 

 dead bacterial cultures, known as vaccines. It is only fair to state 

 that many years earlier Wright had used heated typhoid bacteria in vac- 

 cination against this disease. It is not my purpose to discuss here 

 either the methods of employment or the value of bacterial "vaccines." 



The normal blood of man and other animals contains opsonins or 

 substances which favor phagocytic action with numerous bacteria, and 

 these are increased by proper vaccination. When the normal blood 

 contains no opsonin for a given bacterium, one can be developed by 

 vaccination, that is, by repeated parenteral administration of that 

 bacterium either in numbers too small to endanger life or attenuated 

 or killed by heat. In some instances the bacterium while in the body 

 forms a capsule. This is strikingly true of anthrax and plague bacilli. 

 The apparent purpose of this is self-protection. Not all bacteria which 

 are taken into phagocytes are killed. However, even in these cases 

 phagocytosis is protective to the body. When bacteria undergo com- 

 plete disruption outside a phagocyte, its poisonous content is set free 

 and exerts its deleterious effects. The inclusion of dead bacteria is 

 protective for the same reason. When there is no phagocytosis and 

 the bacteria are rapidly split up by a bactericidal serum, the greater 



