THE DEFENSIVE MECHANISMS OP THE BLOOD. 153 



destroy them. Yet, in the defense of the organism, they 

 take a part which is no less important than that of the 

 phagocyte. 



In very simple forms of life the cells of the alimentary tract 

 both ingest and digest the food material. In higher forms the 

 cells of the alimentary tract secrete the fluids which digest the 

 food. In the one case the digestion is intra-cellular, and in the 

 latter, extra-cellular. In the same way we find the blood leu- 

 cocytes able both to destroy and to digest substances by intra- 

 cellular action, and also sharing with other cells of the body the 

 power to secrete substances into the blood plasma which have the 

 power of destroying the organisms or toxic material. 



Opsonins. Normal blood serum has a very strong destruc- 

 tive influence on most species of bacteria, whether they are patho- 

 genic or not. This ability is not possessed to the same extent by 

 the blood plasma. The difference is explained by the fact that 

 in the process of coagulation the white blood cells are broken 

 down and liberate their bactericidal bodies. Extracts made of 

 leucocytes have this same effect, but the reaction is much more 

 rapid in the presence of blood plasma or serum. The co-oper- 

 ation on the part of the plasma or serum is explained by the 

 presence of some substance in solution which enables the leu- 

 cocytes the more readily to attack the bacteria. 



That some such substances also aid in the phagocytic action of 

 the leucocytes is indicated by the fact that the white cells ingest 

 bacteria much more readily in blood serum than in normal saline 

 solution. These substances are known as opsonins, and are char- 

 acteristic for each individual organism which stimulates their 

 production. At the beginning of an infective process, in which 

 the phagocytosis is very active, each leucocyte may be able to at- 

 tack only one or two bacteria ; later in the disease, however, when 

 the opsonic power has been increased for the infective agent, the 

 leucocytes may be able to ingest a much larger number without 

 injury to themselves. The opsonic index is a figure expressing 

 the ratio of the number of pathogenic organisms of a certain 

 kind that a normal leucocyte can ingest in serum, to that which 

 the same leucocyte can ingest in the presence of the serum of a 



