THE OPSONIC INDEX 257 



free of serum by centrifuging with saline, then added 

 to a suspension of leucocytes, and the mixture finally 

 incubated, phagocytosis takes place. That is to say, the 

 presence of serum is unnecessary to induce phagocytosis, 

 provided the bacteria have previously been acted upon 

 with serum ; the serum acts in some way on the bacteria, 

 rendering them suitable prey for the phagocytes. This 

 thermolabile serum feast-preparer is termed " opsonin " 

 by Wright and Douglas (from a Greek word meaning 

 " to cater for "). 



They have also shown that during the process of active 

 immunisation the opsonic value of the serum is increased, 

 and they have succeeded in demonstrating this opsonic 

 immunity for a number of infections, such as the staphylo- 

 coccic, Malta fever, pneumococcic, tuberculous, etc. 

 By comparing the amount of phagocytosis, i.e. the average 

 number of bacteria ingested per leucocyte, induced by the 

 patient's serum towards the organism of the infection 

 with that similarly induced by normal serum, a figure is 

 arrived at which is known as the " opsonic index." This 

 is obtained by finding the ratio of the numbers of bacteria 

 ingested by, say, one hundred leucocytes with the two 

 sera and taking the figure for the normal serum as 1. In 

 an acute infection, the corresponding figure for the 

 patient's serum will probably be 0-4-0- 8, which would be 

 the opsonic index in this instance (for details of the method, 

 see p. 259). 



In subacute and chronic localised infections the opsonic 

 value of the serum is usually diminished, occasionally 

 increased. In acute infections the index will, as a rule, 

 be low ; in chronic infections which are not strictly 

 localised, e.g. tuberculosis, the index will sometimes be 

 low, sometimes high. A low index generally indicates an 

 infection, or a low power of resistance to the particular 

 organism, or that a chronic but quiescent infection 



M.B. 17 



