SUSCEPTIBILITY TO OPSONIFICATION 189 



of complement, whereas the opsonins, although active to some extent 

 in the absence of complement, are far more so if a complement is present. 

 In this respect they resemble amboceptors, or receptors of the third 

 order, opsonins in normal and immune serums representing respectively 

 normal and immune bacterial amboceptors. One objection to^this 

 view of their structure is their activity, however slight, when the ther- 

 molabile substance is removed by heating, unless the amboceptors are 

 complemented by an endocomplement, as from the bacteria themselves. 



At the present time, therefore, not a few observers doubt that opso- 

 nins exist as true and separate antibodies, and are inclined to regard 

 thermolabile opsonin (largely the opsonin in fresh normal serum) as a 

 complement, and thermostabile opsonin (largely immune opsonin or 

 bacteriotropin) as an amboceptor; it would appear that either alone, 

 but more especially the latter, may facilitate phagocytosis to some ex- 

 tent. This process is, however, much more marked when both sub- 

 stances are acting in unison. While it is true that the bacteriolysin 

 and opsonin content of a serum do not run parallel, our methods for 

 measuring these are not entirely satisfactory; both intracellular and 

 extracellular lysis may be mere differences in degree, depending upon 

 the nature of the bacterium or the concentration of the antibodies, 

 rather than upon separate and distinct antibodies. 



Source of Opsonins. Little is definitely known regarding the source 

 of opsonin. Thermostabile opsonin that which is increased by arti- 

 ficial immunization or during disease, and is largely in the nature of an 

 amboceptor is probably a product of general cellular activity, and 

 especially of the local cells at the site of infection. Thermolabile opsonin 

 largely the opsonin occurring in normal serum, and in the nature of a 

 complement is probably a product of the leukocytes and other cells as 

 well, as it has never been proved that the leukocytes are the sole source 

 of the complements, as Metchnikoff would have us believe. 



Susceptibility to Opsonification. As previously stated in the chap- 

 ter on infection, not all bacteria are equally susceptible to opsonifica- 

 tion. As a general rule, recently isolated and virulent microorganisms 

 resist the influence of opsonins until they have undergone culture sev- 

 eral times. This resistance may be due to capsule formation, thicken- 

 ing of the ectoplasm, actual self-immunization of the bacterium, or the 

 influence of endotoxins as a protective means against the antibodies of a 

 host, all of these being weakened or lost upon artificial culture-media. 



Effect of Opsonins on Bacteria. We know nothing definite regard- 

 ing the manner in which opsonins prepare bacteria for phagocytosis 



