280 president's address — section l. 



Now, what is thei nature of this combination of antigen -anti- 

 body and complement ? 



Ehrlich's theory of a chemical nnion has already been discussed. 

 As against this view, Bo^rdet and his co-workers have put the view 

 which may now be said to prevail, namely, that complement is 

 fixed not iDy antibody alone, nor by antigen alone, but only by the 

 antigen-antibody combination. In other words, the' action of the 

 antibody, developed as a result of the immunity, is to combine 

 with the antigen, and render it able to adsnrh complement. An 

 analogy is seen in the action of the agglutinins in an immune 

 serum. These agglutinins combine with the bacteria against which 

 they have been developed, and renaer the bacteria able to adsorb 

 f-alt from the saline solution in which they are suspended. Without 

 salt bacteria are not agglutinated by a serum, however rich it 

 may be in agglutinins. But add salt to the mixture, and you will 

 add an electrolyte^, which, adsoibed by the bacteria-serum combina- 

 tion, causes the agglutination. 



So complement fixation is really an adsorption of complement 

 by colloidal substances, and this theory explains many curious 

 phenomena of serum reactions. Serum is itself a complex body in 

 a state of very unstable colloidal equilibrium, and very slight and 

 subtle alterations can upset thb molecular adhesion which is 

 normal. 



And now, lastly, let us consider briefly, one of the most difficult, 

 but at the same tinie most intereisting, of the phases of immunity, 

 namely, A )iaph//I<i.ris. 



A Hfiph 1/1 axis. 



Anaphylaxis, -the name given by Richet signifying the reverse 

 of prophylaxis, refers to a condition of extreme sensitivity or sus- 

 ceptibility to the injection of certain substances. If, during the 

 attempt to immunize an animal, a first injection of a particular 

 antigen is given, and then after an interval of ten days or more 

 a second dose is given, a peculiar set of symptons is often pro- 

 duced, a reaction soi severe as tO' be often fatal, and that v/ithin 

 a few minutes or a few hours. Thus, the first injection, intended 

 to make the animal more refractory to infection, has apparently 

 made it hypersensitive. 



The first observation recorded which may fairly be thought to 

 be of this nature is on© of Magendie, in 1839, describing the sud- 

 den death of dogs injected repeatedly with egg albumen. Several 

 isolated observations by other workers were brought to light after 

 the publication of Arthus in 1903 with regard to the effects of 

 repeated injections into rabbits of horse serum. Theobald Smith 

 also recorded the great susceptibility of guinea pigs used for the 

 standardization of diphtheria toxin to a subsequent injection of 

 normal horse serum. Since that time the subject has been investi- 

 gated by very numerous workers, and the facts elicited have done 

 much to increase our knowledge of immunity generally. 



