540 IMMUNITY 



phagocytes. To this they gave the name of " opsonin " (vide 

 pp. 122, 519). There is no phagocytosis of cocci by leucocytes 

 washed in salt solution ; normal serum heated to 55 C. is also 

 without effect in inducing this phenomenon. They could not 

 demonstrate any effect of the opsonin on the leucocytes. On the 

 other hand, if bacteria be exposed to the fresh serum, and they 

 be freed from the excess of serum and then exposed to phago- 

 cytes also washed free from serum, they will be readily taken 

 up by the cells. It has been abundantly shown that the opsonic 

 action of the serum is increased by the process of immunisation 

 against an organism, and the opsonic index represents the 

 degree o'f immunity in one of its aspects as already explained 

 (p. 122). The matter has, however, become complicated by the 

 circumstance that in an immune-serum an opsonin may still be 

 present after the serum is heated at 55 C., as has been shown 

 by Dean and others ; and Muir and Martin have shown that 

 this thermostable immune-opsonin (bacteriotropin of Neufeld) 

 has all the specific characters of anti-substances in general. On 

 the other hand, they have found that the thermolabile opsonin 

 of a normal serum has quite different properties. For example, 

 when a normal serum is tested on a particular bacterium, the 

 opsonic effect on that bacterium may be removed by treating the 

 serum with other bacteria; in other words, the thermolabile 

 opsonin of normal serum does not possess the specific character 

 of the opsonin developed in the process of immunisation. They 

 have also found that various substances or combinations of sub- 

 stances which act as " complement absorbers " also remove the 

 opsonic property from a normal serum, while they have no effect 

 on an immune-opsonin. 



That this thermolabile normal opsonin can act in a non- 

 specific way is shown by the fact that particles of car- 

 mine and other substances become opsonised by the action 

 of normal serum. It is, however, to be noted that in certain 

 cases there have been found in a normal serum traces of sub- 

 stances which can be activated by thermolabile opsonin after 

 the manner of immune-body and complement (as seen in the 

 haemolytic action of a normal serum (p. 538) ; to this extent the 

 opsonic effect of a normal serum may have some degree of 

 specificity. From this and other facts some observers have 

 attempted to explain the whole of opsonic action according to 

 the scheme of immune-body and complement as seen in h&mo- 

 lysis. This, however, is not justifiable, ' since normal thermo- 

 labile opsonin can, as we have seen, act by itself, as can also 

 the specific immune-opsonin after normal opsonin has been 



