THE AMERICAN NATUBALIST [Vol.LYI 



duction of bacterial immobilizers or solvents termed 

 h act etioly sins, or sometimes to agglutinating substances 

 termed agglutinins whicli clump bacteria of the species 

 used in their production, if the two are brought together 

 in the blood-serum of the animal into which the bacteria 

 were originally introduced. Likewise a tissue of one kind 

 of animal injected into the circulation of another induces 

 the formation of antibodies of various kinds such as 

 precipitins which form a precipitate when the blood-serum 

 of the treated animal and an extract of the special tissue 

 used are brought together in vitro; or other antibodies 

 termed cytotoxins or cytolysins which possess a specific 

 toxic or solvent action for the kind of protein used in 

 their production. The alien substance employed to pro- 

 duce antibodies is commonly called the antigen. 



In this connection, the phenomenon of anaphylaxis 

 should perhaps also be mentioned. Anaphylaxis is a 

 name given by Richet to designate a highly supersensi- 

 tive state w^hich, after a period of incubation, an animal 

 develops toward certain protein substances that w^ere 

 practically harmless on first injection. Sometimes, par- 

 ticularly in guinea pigs, death results. The sensitizing' 

 dose for the production of anaphylaxis may be very 

 small ; one millionth cubic centimeter of horse-serum, for 

 example, has been known to render guinea pigs sensitive. 

 The reaction is specific; for instance, an animal sensitized 

 to sheep-serum, though reacting violently to this antigen, 

 displays little or no hypersusceptibility to other sera. 



In the main all of the immunological reactions sliow a 

 considerable degree of specificity; the antibody will react 

 fully only with the particular kind of protein used as 

 antigen. The specificity is not absolute, however; a 

 milder reaction may be obtained with homologous 

 proteins of related species, the extent of the reaction 

 being determined l)y the nearnoss of relntioiisliii) of the 

 species to tlinl from wliidi tlir oiM-iiinl a?iti-vn \va< ob- 

 tained. Siiiii!ai'l\- ^vitll hactci-ia. the I'cactic^n 1- in tlie 

 main specific, altlioiiu'li xi-calloil ufoiit) rcactioiH iiia>- a]v 

 pear. The <vv\\m of an aiiimal iniiimiii/.<Ml a-aii"i^t 'ty- 

 phoid, for exain|.i('. may not only au-ulntinato /larill'ii.^ 

 typhosus but may also show this reaction in a less doirree 

 with such related forms as the colon bacillus. Thus, ir- 



