6o 4 SCIENCE PROGRESS 



blood is poor in protective substances. The presence of these 

 protective substances in the blood can be demonstrated, and 

 their amount measured by methods introduced by Wright and 

 Douglas. 1 These observers estimate the amount of phagocytosis 

 which occurs when white blood-corpuscles, bacteria, and blood 

 serum are mixed together. 



Phagocytosis is the power of ingesting foreign bodies 

 which is possessed by white blood-corpuscles (leucocytes). It 

 was first observed by Haeckel in 1862, in the leucocytes of a 

 mollusc. 2 Leucocytes can ingest bacteria, and it was long 

 contended by Metchnikoff, and his school, that immunity 

 depended on this power. The experiments of Wright and 

 Douglas proved that something further is required. II 

 leucocytes are washed free of their serum and mixed with 

 bacteria, phagocytosis does not occur, or only to a very small 

 extent. If to this mixture the serum of a healthy man is added, 

 the leucocytes will ingest a number of bacteria, and an average 

 per leucocyte can be worked out. 



Phagocytosis depends on some property of the serum, some 

 substances which act on the bacteria so as to render them 

 capable of ingestion by leucocytes. Wright has called these 

 bodies " opsonins." If the experiment mentioned is repeated, 

 substituting for the serum of a healthy man that of a person 

 suffering from a local infection by the bacterium in question, it 

 is generally found that his serum has less power of preparing 

 bacteria for ingestion, and that a smaller number of bacteria 

 are taken up by the phagocytes. In practice the average 

 number ingested after using normal serum is regarded as 

 unity, and the number ingested after using the patient's serum 

 is expressed as a fraction of the normal. This fraction Wright 

 calls the "opsonic index." 



By this means it is possible to estimate the patient's resist- 

 ance to the invading organism, and whether benefit is likely 

 to follow the exhibition of a vaccine. The explanation of the 

 low index in local infections is that the focus is to some extent 

 shut out of the blood stream, that the protective bodies have 

 been exhausted in their unsuccessful attempt to destroy the 

 focus, and that bacteria do not pass into the blood and 

 stimulate the formation of anti-bodies. It is believed that these 



1 Wright and Douglas, Proc. Roy. Soc, vol. Ixxii. 



2 Metchnikoff, Lemons sur P Inflammation comparee. 



