190 CHEMISTRY OF THE IMMUNITY REACTIONS 



SO different from the normal proteins of the same animal that they will 

 act as an antigen when present in the blood of that animal, or an- 

 other of the same species, from which they were derived. Of the 

 natural proteins of serum the giobulins are much more active precipi- 

 tinogens than the albumins. In general the more foreign the protein, 

 the greater the amount of precipitin; closel}^ related animals, e. g., rab- 

 bit and guinea-pig, produce little precipitin for one another's pro- 

 teins. This indicates distinctly that difference in species depends 

 upon or is associated with difference in chemical composition of the 

 proteins. DiffereJit sioecies of animals have very different capacity 

 for producing precipitins, rabbits producing active sera, while guinea- 

 pigs can produce but feebly precipitating sera. Cantacuzene ^'^ be- 

 lieves that precipitins are formed chiefly in the lymphoid tissues and 

 bone marrow, and that the mononuclear macrophages are most active 

 in their formation.*"'' Only proteins can produce precipitins; when 

 split to the peptone stage they lose this property, but the proteins of 

 serum resist tryptic digestion a long time before losing their precip- 

 itinogenic property,"*^ which is destroyed much more quickly by pep- 

 sin-HCl mixtures. The precipitate itself is very resistant to disin- 

 tegrative agencies, including putrefaction (Friedberger),"^ but is 

 soluble in dilute acids and alkalies. It has the power of binding 

 complement (Gay^-) and if the complement causes solution of the 

 precipitate, poisonous substances are formed (Friedberger). Ex- 

 cess of antigen prevents the formation of precipitate, or redissolves 

 it, but excess of antiserum has no effect. Since both reacting sub- 

 stances are colloids tliey follow the laws governing other mutually 

 precipitating colloids, and precipitation occurs only when they are 

 brought together in concentrations that lie within definite zones of 

 relative proportions. It is, of course,' perfectly possible to have a union 

 of precipitin and antigen without anj^ visible precipitate occurring, 

 since the product of the reaction is not necessarily insoluble under all 

 conditions; in this case the occurrence of a reaction must be demon- 

 strated by some other method, e. g., the complement fixation reaction. 

 No precipitins can be secured against lipoids or other non-protein sub- 

 stances. Possibly precipitins can be produced for closely related sub- 

 stances with molecules approximating in size the protein molecule, e. g., 

 certain substances present in supposedly protein-free filtrates of bac- 

 terial cultures. As with the agglutinin reaction, electrolytes must be 

 present or precipitation will not occur. Neither the precipitin nor the 



so Ann. Inst. Pasteur. 11)08 (22), 04. 



8»a Spleen tissue eiiltivated artificially in tlie presence of liorse serum jiroduces 

 specific precipitins for liorse serum, and tissue from the spleon of a guinea pisr 

 that has received injections of horse serum also develops precipitins for horse 

 serum wlien grown in cultures (Przygode, Wien. klin. Wo<h., 1014 (27), 201). 



90 Fleischmann, Zeit. klin. Med., fOGG (50), 515. 



niCcnt. f. Bakt., 1007 (4.3), 400. 



'••-'See Univ. of falif. Publ. Pathol., 1011 (2), 1. 



f 



