Protein Specificity lOy 



steiner and others, aid in defining a particular agglutinin group; adsorption 

 on cells (blood or sperm) of any one species removes the agglutinins for other 

 species of the same group but not for more distantly related groups. 1 he 

 seminal fluids and body fluids of various invertebrates contain natural heteroag- 

 glutinins for sperm and for vertebrate red blood cells. ^^ 



Immune Precipitins. The immunological reactions most widely used in 

 indicating degree of difference among specific proteins of different species are 

 precipitm tests. Nuttall •*" applied precipitin tests to sera and egg albumin 

 from many species of vertebrates and indicated the usefulness of the lest in 

 taxonomy. A standard procedure at present is to build up antibodies in a 

 rabbit by injecting known amounts of an antigenic protein. The rabbit serum 

 is then mixed with the protein to be tested, and the maximum dilution of this 

 protein giving a precipitate is noted. Homologous serum or egg albumin, that 

 is, protein from the same species as the antigen, gives a maximum precipitate, 

 and more distant taxonomic relations tend to be indicated by less precipitate. 

 Results express the greatest dilution for heterologous precipitate as percentage 

 of the threshold for the homologous reaction (titer). I'o be highly signiticant, 

 reciprocal heterologous tests should be performed, that is, test of antiserum x 

 against antigen y, as well as test of antiserum y against antigen x. Unfortu- 

 nately, reciprocal tests are not always in close agreement. Quantitative measure 

 ments of the precipitate are made with a turbidometer, by a "ring test," or by 

 measuring the change in nitrogen concentration after precipitation. Detailed 

 descriptions of methods, and reviews of the literature are given by several 

 authors.8- 9- n- 12. 19, 51 



Precipitin antibodies can be built up against other tissue proteins besides 

 the blood globuHns. Such antibodies show some species and organ specificity. 

 Proteins of the lens of the eye have antigenic components which are very 

 similar, even in different classes of vertebrates.-" Proteins which are indis- 

 tinguishable chemically can be separated immunologically. 



The degree of similarity of serum proteins among species of mammals as 

 indicated by serological reaction agrees well with taxonomic relationships. 

 Sheep blood is close to that of beef, farther from that of pig and horse, and 

 very far from that of dog. " The superfamilies of rodents are separated readily, 

 families and sub-families less readily. '^'' The chicken is close to the turkey, 

 farther from the guinea hen, and still farther from the duck, as indicated by 

 reactions of either serum proteins ^'* or egg albumin. •'"' In the ovalbumin 

 the protein responsible for homologous reaction differs from the protein respon- 

 sible for heterologous precipitation. ^^ In the genus Rana the species cates- 

 biana and clatnitans are close serologically, whereas pipiens is distant.'-* 

 Among urodeles the primitive genus Cryptobranchus is far from Amphiiima, 

 Siren, and Necturus, and these latter genera are nearlv equally separated 

 (Fig. 27). 



The relations among prochordates and other inxertebrates have been exam- 

 ined by comparing precipitin titers for several antibodies with proteins from 

 a wide variety of animals. **" The percentages of heterologous titer are low 

 between ascidians and balanoglossids, but these titers are higher than between 

 either of these groups and any other phylum. On the basis of such comparison 

 the prochordates are more closely related to echinoderms than to polychaetes 

 and to Phascolosoma. Polvchaete antisera give low reactions to ascidians and 



