OPSONINS OR SAUCE FOR GERMS 355 



strated by experiment. Our main means of defence against 

 microbial disease, says Metchnikoff — tiiough cleanliness 

 and precaution against access of microbes are all very well 

 in their way — is the activity of our phagocytes. Now it 

 appears that just as in the other cases I have been con- 

 sidering, so in the production of " relish," the power to 

 produce it resides in the blood (and perhaps the cells of 

 its vessels), but is not set at work until the enemy is in 

 the blood. Suppose there is an infection, an invasion of 

 the blood and tissues by one or other disease-causing 

 microbe. Gradually if the body is healthy the '' relish " is 

 produced and becomes attached to the invading microbes. 

 The phagocytes swallow them greedily and make an end 

 of the invasion. 



It is proved that this aroused avidity of the phago- 

 cytes is due to no change in the phagocytes themselves ; 

 since if they are transferred to the serum of a normal 

 man they show no such predilection for the special 

 invading microbe. The " opsonin," or " relish," is some- 

 thing exuded into or produced in the blood fluid when 

 the attacking microbe arrives. It attaches itself to 

 them : that is the essential fact. In many of us the 

 phagocytes are not at a given moment so " avid " of this 

 or that disease-microbe as they should be in order to pro- 

 tect us from its multiplication and poison production. But 

 it is found that by injecting boiled and cooled (therefore 

 dead) microbes of a particular kind into the blood of a 

 man, you can start the production of the " relish " appro- 

 priate to that kind. The dead microbes answer this 

 purpose ; they excite the production of the opsonin appro- 

 priate to them and yet are not themselves dangerous, since 

 they are dead. When subsequently (or possibly concur- 

 rently in small quantity) living microbes of the same disease 

 enter the blood, the opsonin is ready for them. They are, 

 to put it picturesquely, like oysters at the oyster-bar, 



