838 IMMUNOLOGY 



serum globulins slightly modified (see Wells, 1929; Marrack, 1938). 



Antibodies are variously named according to the effect produced when 

 mixed with antigen. The antibody is a precipitin if it produces a pre- 

 cipitate on mixing with a soluble antigen (precipitinogen). It is an 

 agglutinin if it induces clumping or agglutination of cellular antigens 

 (agglutinogens), such as Bacteria, Protozoa or blood cells. It is an 

 opsonin if it sensitizes the antigen and makes it more readily ingested 

 by phagocytes. It is a lysin if it sensitizes cellular antigens so that, on the 

 addition of a thermolabile component of normal serum known as com- 

 plement or alexin, the cell undergoes death and lysis, during which many 

 of its internal substances diffuse through the cell membrane. In both of 

 the foregoing processes, the antigen is first sensitized by antibody. After 

 such a preparation, it is then lysed by intracellular enzymes (phagocyto- 

 sis) or by extracellular enzymes (lysis) (see Wells, 1929). The antibody 

 is known as an antitoxin if it neutralizes the biological action of a toxic 

 antigen (exotoxin). Definite antitoxins and exotoxins have not been 

 demonstrated in protozoan infections (see W. H. Taliaferro, 1929). 



An increasing number of immunologists accept the unitarian view- 

 point that the introduction into the body of a single antigen results in 

 the formation of a single antibody, which is an agglutinin, precipitin, and 

 so forth, according to the nature of the antigen or the particular method 

 of testing. This does not mean that a complex cell will not contain many 

 different antigens. Furthermore, a given antibody in a specific infection 

 may act as one type of antibody, and not as another, because of the posi- 

 tion of various antigens on or within the cell (see Topley, 1935). 



Finally, an antibody-like substance is known as ablastin if it inhibits 

 the reproduction of organisms when mixed in vivo. So far, it has been 

 demonstrated only for certain nonpathogenic trypanosomes. Like other 

 antibodies, it is associated with the globulin fraction of serum and is 

 passively transferable, but differs from them in that it has no in vitro 

 affinity for its specific antigen. In the latter respect, it appears to re- 

 semble certain nonabsorbable antibodies reported in bacterial, virus, 

 and worm infections. 



The role of antibodies is studied by in vivo protective (passive trans- 

 fer) and curative tests and by in vitro studies involving various serologi- 



