242 SCIENCE PROGRESS 



Opsonins. — These substances, discovered recently by Wright 

 and Douglas, 1 are chemical bodies which act upon the offending 

 micro-organisms and render them capable of being ingested by 

 the white corpuscles. They do not kill the bacteria, but only 

 prepare them for ingestion by the phagocytes. 



This research, while confirming the important part played 

 by the phagocytic cells of the blood in bacterial infections, has 

 shown that the bacteria must be acted on by the opsonins 

 before they can be ingested -and ultimately destroyed by these 

 cells. 



The opsonins are thermolabile substances, being destroyed 

 by exposure for ten minutes to a temperature of 6o° C, and also 

 by sunlight. In contrast to the bactericidal substances and 

 agglutinins, which have been demonstrated only in connection 

 with isolated species of organisms, opsonins exist for each 

 species of bacteria, and they can be severally extracted from 

 the serum by a process of saturation ; they are " specific for the 

 different strains of bacteria." 2 The discovery of these substances, 

 and the technique by which they can be quantitatively estimated 

 to an accurate degree in a small sample of the blood taken from 

 the finger, has led to an extremely important research, which 

 has thrown light on many dark problems of medicine. 



Immunity, using this term as the equivalent of increased 

 resistance of an organism to bacterial infection, can be achieved 

 in two ways. To one of these Ehrlich has given the name 

 "active immunity," to the other " passive immunity." 



Active immunity may be defined as the increased resistance 

 to bacterial infection which is obtained by the inoculation of 

 bacterial vaccines. Passive immunity may be defined as the 

 condition of increased resistance which is obtained by transferring 

 to an organism protective substances elaborated in the organism 

 of another animal which has been actively immunised. 



Of these two kinds of immunisation, passive immunity, 

 although dependent upon active immunity, was the first as 

 regards its application to medicine. For this reason serum- 

 therapy will be dealt with first, and vaccine treatment will 

 receive consideration later on in this paper. 



Serum-therapy. — Anti-sera are of two kinds : (a) Antitoxic, 

 (b) Anti-bacterial. The former, of which the diphtheria and 



1 Proc. Roy. Soc, vol. 72, 1903. 



3 Bulloch and Western, Proc. Roy. Soc, 1906. 



