ANTIGENS AND ANTIBODIES 21 



have considerable diagnostic importance, to which at- 

 tention will be called later. 



(d) Lysins, — Lysins are among the most impor- 

 tant of all antibodies, both from the stand-point of 

 protection against disease and also from the fact that 

 many diagnostic tests are based upon their action. 

 Lysins are antibodies that bring about the solution of 

 cells, and the term cytolysin may therefore be applied 

 to them all. Lysins which bring about the solution of 

 bacteria are termed bacimwlysins ; those which cause 

 solution of red blood-cells are known as hcEinolysins, 

 Thus, the terms cytolysis, bacteriolysis and hcemolysis 

 should be readily understood. Lysins are formed as 

 the result of introduction of bacteria or of cells of an 

 animal of an alien species into the body. Thus, the 

 red blood-cells of the sheep, when introduced into the 

 body of the rabbit, produce antibodies or h^emolysins 

 in the rabbit's blood-serum, which when brought sub- 

 sequently in contact with sheep's corpuscles under cer- 

 tain conditions will dissolve the latter. 



{e) Isocytolysins. — It has been found that lysins 

 may not only be produced against the cells of an ani- 

 mal of a foreign species (heterolysins) but also that 

 the introduction into the body of cells of an animal of 

 the same species will cause the formation of specific 

 cytolytic antibodies (isocytolysins). 



(/) Opsonins, — Opsonins are substances present 



