OPSONIN THEORY OF IMMUNITY. 21 



normal rabbit on the addition of normal rabbit serum, have only 

 slight phagocytic action for the streptococcus. If, however, 

 leucocytes from a normal rabbit, or from a streptococcus-immune 

 rabbit are added to the serum from an animal vaccinated 

 with streptococci, the leucocytes actively engulf and destroy 

 the streptococci. When streptococci, able to produce erysipelas 

 in a normal rabbit, are injected under the skin of a rabbit vacci- 

 nated against streptococci, disease is not produced. This pro- 

 tection is due, according to these investigators, especially to the 

 ability of the leucocytes to destroy streptococci. They attribute 

 the increased phagocytosis to an action of the immune serum on 

 the leucocytes. 



The results obtained and conclusions arrived at by Denys 

 and LeClef were repeated by Bordet, who was not able to verify 

 the results obtained by these investigators. Mennes found that 

 immunity to Mic. pneumoniae depends on the action of serum 

 which produces active phagocytosis but was not certain that 

 this action is due to stimulation of the leucocytes. 



In 1903 Wright and Douglas pointed out that there are cer- 

 tain substances in serum that so affect bacteria that they are 

 more easily taken up and disposed of by the leucocytes. This 

 substance they called "opsonin." They found opsonin present in 

 normal and immune serum and from their investigations decided 

 that the amount present in serum is variable, and can be increased 

 or decreased by the injection of killed cultures of bacteria. To 

 determine the amount of opsonin present in blood they modified 

 the method of estimating phagocytic power introduced by Leish- 

 man in 1902. According to their method the average number 

 of bacteria taken up by leucocytes, when bacteria, leucocytes and 

 serum are mixed together and allowed to remain together for a 

 certain time, is determined. The ratio of the average number 

 of bacteria per leucocyte when patient's serum and serum from 

 a healthy individual determine the phagocytosis, they called the 

 opsonic index. The methods employed in the determination of 

 this opsonic index will be taken up later. % 



In 1904 Neufeld and Rimpau, entirely independently of 

 Wright and Douglas referred to the two well-known elements, 

 antitoxin and bactericidal substances, in immune serum and stated 

 that in anti-streptococcic and anti-pneumococcic serum they had 



