106 Comparative Animal Physiology 



are immunological reactions. The y-globulin traction of serum proteins 

 contains antibodies of many sorts, natural and acquired, which are highly 

 specific in their reactions with other proteins, with foreign blood cells, and with 

 bacteria. 



Most proteins and some polysaccharides can act as immunizing antigens, 

 in that they can induce the production of antibodies in animals whose circu- 

 lating fluid does not contain the same protein or polysaccharide. The anti- 

 bodies produced react with their specific antigens, causing agglutination of 

 cells, lysis of cells, or precipitation of foreign proteins. Immune reactions of 

 mammals are discussed in textbooks of immunology.'^ Development of anti- 

 bodies, such as agglutinins, lysins, precipitins, or others, is important in defense 

 against infectious agents. Immunity reactions are best known in mammals, 

 but they have been studied in most groups of animals. Among invertebrates 

 many examples of natural immunity are known. Caterpillars and other insects 

 have been successfully immunized against pathogenic organisms, -'^ although 

 failure to produce antibodies is reported in echinoderms, annelids, and mol- 

 luscs. Phagocytosis is an important immune process in invertebrates and is 

 shown by many types of cell. 



Naturally Occurring and Immune Agglutinins. The sera of various verte- 

 brates normally contain agglutinins that act on the red blood cells of certain 

 other species or on the red cells of certain groups of individuals of the same 

 species but of nonhomologous strain. A fraction of a milliliter of serum from 

 a fish injected into a rabbit may cause the death of the rabbit, owing to cell 

 agglutination. Landsteiner '^^ has used agglutination to separate the blood 

 types among humans into four groups, O, A, B, and AB. Each individual 

 contains in his serum the natural antibodies for the antigen types which are 

 absent from his erythrocytes. Thus transfused red cells are agglutinated by 

 serum antibodies of different type from the cells. The chimpanzees, gibbons, 

 and orangutans-"' ''^^' '^^ can each be classified under two of the human types, 

 whereas in the old world monkeys, Cercopithecediae, no agglutinogens of 

 human types are found, and in new world monkeys, Platyrrhina, and lemurs 

 there is one blood type which is similar to type B in man. 



In addition, there are two antigens in human erythrocytes, M and N, for 

 which there are normally no antibodies; the distribution of M and N antigens 

 is hereditary and independent of the A and B antigens. A third antigen, Rh, 

 occurs in erythrocytes, and Rh-positive red cells can induce the production of 

 Rh antibodies in a person negative for the Rh factor. This antiserum can then 

 cause agglutination, followed by hemolysis of Rh-positive cells. When the 

 three classes of hemagglutinogens, each independent of the other, are con- 

 sidered, the total number of genetic blood types is large; furthermore, sub- 

 groups are known. The three factors are distributed genetically, and certain 

 races display characteristic combinations. Agglutinating factors in the blood 

 of various species of mammals and birds are favorite subjects of genetic studies. 



Besides the agglutinins which clump red blood cells, there are agglutinins 

 for other blood cells and for sperm. These have been found not only among 

 vertebrates but also among invertebrates. For example, serum of the spiny 

 lobster, Panulirus inierruftus, contains at least ten heteroagglutinins for sperm 

 or blood cells of a variety of species. '^''^ ■*** These agglutinins occur in serum 

 proteins other than hemocyanin. Cross adsorption tests, as used by Land- 



