484 IMMUNITY 



leucocytes depended on a body in the normal serum which 

 became fixed to the cocci and made them a prey to the 

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

 pp. 194, 261). 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 immumsa/EToh 

 against an organism, and the opsonic index represents the degree 

 of immunity in one of its aspects as already explained (p. 111). 

 The matter has, however, become complicated by the circum- 

 stance 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. Some observers consider that this opsonin is simply 

 an immune-body, but the results brought forward by others would 

 point to their being different substances, at least in certain cases, 

 notably in haemolytic sera. We are, however, probably safe in 

 saying that the thermostable opsonin of an immune-serum is a 

 true anti-substance\ possessing the specific characters of anti- 

 substances in general and comparable in this respect and in its 

 mode of production with an agglutinin. Muir and Martin have, 

 however, found that the thermolabile opsonin of a normal serum 

 has different characters. 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 substances 

 which act as "complement -absorbers" also remove the opsonic 

 property from a normal serum, while they have no effect on an 

 immune-opsonin. According to this view the opsonic effect of 

 the unheated serum of an actively immunised animal or person 

 would represent the sum of the effects of the two kinds of 

 opsonin. 



Further study will be necessary before the exact relationships 

 of these substances are fully understood, and other questions with 

 regard to them have as yet scarcely been touched upon. 

 Increased phagocytic action had long been known by the work 

 of Metchnikoff to be associated with the development of active 



