328 INFECTION AND IMMUNITY. 



ining the dilution point at which the number of 

 bacteria taken up by the leucocytes equals the 

 number taken up without the presence of serum. 

 This dilution point is sometimes called the point 

 of opsonic extinction. The phagocytosis taking 

 place without the influence of serum is known as 

 spontaneous phagocytosis. 

 specificity of There has been considerable conflict of opinion 

 &g to w ] ie ^ er ^ ere are specific opsonins in normal 

 serum for different varieties of cells or one opsonic 

 substance capable of acting on a variety of cells. 

 Hektoen concludes from his own studies and those 

 of others, consisting of specific absorption experi- 

 ments and observations on the specific fall in 

 opsonic power following injection of specific anti- 

 gen, and from other experiments, that normal 

 serum contains specific opsonins which are capable 

 of specific absorption and which are the same 

 substances which are increased to form the im- 

 mune opsonin. 



The immune opsonins are easily demonstrated 

 by absorption experiments to be highly specific. 



Hektoen and Euediger have shown that normal 

 opsonins are almost completely destroyed or inacti- 

 vated by heat and are therefore thermolabile. The 

 inactive opsonin (opsonoid) by saturating the 

 receptors of bacteria with the haptophore group 

 prevents further sensitization with fresh serum. 



These investigators also show that opsonin may 

 be bound or neutralized similarly to complement 

 by solutions of various salts. 



The nature of immune opsonins has been the 

 subject of much discussion. As was stated before, 

 immune opsonins resist a temperature of from 56 



