president's address — SECTION L. 269 



and in both dog and fowl the immunity is due to the power ol 

 phagoc} tosis. Again, in the case of animals naturally susceptible 

 to certain diseases and then vaccinated against them, it is remark- 

 able that the bactericidal power of the blood serum of these im- 

 munized animals is very small. This is true of streptococci, 

 staphylococci, pneumococci, diphtheria bacillus and B. pyocyaneus. 

 On the other hand the increase in phagocytosis is remarkable. 



Both active and passive immunization greatly increase the power 

 of phagocytosis, and at first sight the readiest explanation appears 

 to be by survival of the fittest of the phagocytes. But how can 

 this be the case in passive immunization where nO' struggle has 

 gone on in the immunei animal ? Here it must clearly be a 

 property conferred by means of the hyperimmune serum. But 

 does this stimulus act by spurring on the phagocytes to greater 

 efforts or does it so- affect the bacteria as to' make them more sus- 

 ceptible? An experiment of Wright and Douglas makes the 

 point clear. Some leucocytes are treated with highly immune 

 antistreptococcic serum, then washed, and mixed with streptococci, 

 when very feeble phagocytosis occurs. On the other hand, if 

 streptococci are treated with the antistreptococcic seinim, then 

 washed, and mixed with leucocyte^;, phagocytosis occurs with great 

 energy. 



Other bacteria and immune sera show similar results, and 

 Wright and Douglas gave the name of "opsonins" (feast preparers') 

 to these anti-bodies develoj^ed in^ serum as a result of bacterial 

 infection or vaccination. Wright and his collaborators almost 

 desired to make the determination of the opsonins present in a 

 particular serum a measure of immunity, but important as the ?ole 

 of the opsonins in the serum is in preparing the bacteria for 

 phagocytosis, it does net lessen the necessity for phagocytes able 

 to withstand the microbic products, and to digest and destroy the 

 bacteria they engulf. Further, it has been shown in the case of 

 virulent organisms, pneumococci and streptococci, that part of 

 their virulence consists in the development of substances which 

 neutralize the opsonins of the serum, and which therefore may be 

 called anti-opsonins. Not to multiply terms, they appear to 

 belong to the same category as the "aggressins" of Bail. 



A remarkable feature of opsonic action is that the bacteria 

 are acted on in a purely passive manner, quite irrespective, fox 

 example, of whether they are dead or alive. Further, the serum 

 opsonins are able toi prepare for ingestion by leucocytes inert par- 

 ticles such as carbon, melanin, carmine or starch (Wright and 

 Douglas). It is probable that these particles adsorb opsonins (on 

 all the evidence, albuminoid substances) which form a surface 

 having adhesive properties which greatly favours phagocytosis. 

 Phagocytosis itself may be looked upon as a twofold process ; first. 



