SECRETION 59 



substances of colloidal nature produced by animal tissues 

 when introduced into the body in sublethal doses call forth 

 a condition of reduced susceptibility as compared with their 

 effect after an initial dose. This condition is known as immu- 

 nity. While this mode of reaction to toxic substances is in 

 some cases of undoubted utility to the organism, it must be borne 

 in mind that the phenomena of immunity are by no means 

 exclusively of an " adaptive '* significance. Thus the blood 

 of the crayfish when injected into the mouse renders the latter 

 immune against the venom of scorpions, though the crayfish 

 itself is more susceptible to scorpion venom than the mouse. 

 Frog serum injected into the body cavity of the crab specifi- 

 cally protects the latter against the poisonous secretion of the 

 pedicellari^e of certain Echinoderms. The mechanism of 

 immunity is exceedingly complex ; in addition to immunity 

 to poisons produced by secreting glands of larger animals or 

 by micro-organisms the blood of some animals produces 

 specific lysins which directly destroy micro-organisms, and 

 substances, opsonins, which favour phagoc3rtic activity. 



The poisons produced by the nematocysts of Coelenterates 

 illustrate a phenomenon which may be described as the reverse 

 of immunity and is referred to as anaphylaxis. If extract of 

 the tentacles of a sea anemone are injected into a dog intense 

 vascular congestion in the viscera resulting in death follows 

 after a few hours. When a sublethal dose is given, it is 

 found that the administration of a very much smaller quantity 

 of the poison after a certain minimum period of about ten days 

 has much more severe consequences. This supersensitive- 

 ness, according to Richet, involves the co-operation of two 

 factors. From extracts of sea anemone tentacles two toxic 

 substances can be obtained, one congestin when injected first 

 increases the sensitivity of the dog to a subsequent dose of 

 the other known as thalassin. If the order is reversed thalassin 

 acts as an antitoxin, diminishing the sensitivity to the poisonous 

 action of congestin. Neither the phenomena of increased 

 sensitivity or anaphylaxis nor of decreased sensitivity or 

 immunity to specific poisons are, as the important work of Dale, 

 Gunn, and others has shown, due simply to the production of 



