CHAPTER LI 

 THE CHEMISTRY OF ANTIGENS 



SARA E. BRANHAM 



University of Chicago 



An "antigen" is any substance, which, when introduced into an animal body, 

 causes the appearance of certain new and specific properties in the tissues and circu- 

 lating fluids. These changes may be variously demonstrated by the phenomena of 

 agglutination, precipitation, complement fixation, hemolysis, anaphylaxis, phagocy- 

 tosis, and protection against toxin. The reacting factors in the body are commonly 

 termed "antibodies." 



It is by no means the rule for all kinds of antibodies to be demonstrable in the 

 same animal, and even those appearing in a single individual may not be made mani- 

 fest at the same time; frequently a serum that is lacking in agglutinins and precipitins 

 may show marked complement-fixing properties. Nor does a quantitative parallelism 

 exist among those antibodies actually shown to be present; agglutinins may be much 

 more conspicuous than precipitins, complement-filing antibodies may be more in 

 evidence than either agglutinins or precipitins, and all of these may seem to bear little 

 relationship to the antitoxic strength of a serum used. A discussion of the significance 

 of these variations is not relevant to this chapter. 



If a list of all substances which have been employed as antigens were compiled, 

 it would indeed be long. Those most commonly used are very complex materials 

 such as milk, egg white, whole blood, blood serum, blood cells and extracts of blood 

 cells, toxins and venoms, plant pollens, bacteria, bacterial extracts, and tissue ex- 

 tracts of many kinds, even of horse's hoof, cow horn, and human hair! 



What is responsible for the antigenic activity of these very complex materials? 

 Are they fundamentally similar in composition? Is there some essential factor which 

 they possess in common? Or may quite different kinds of compounds be able to cause 

 antibody formation in animals? Investigation of these fundamental questions has 

 been approached chiefly from three angles: (i) the fractionation of the commonly 

 used complicated antigens in order to determine which portions are responsible for 

 their activity and to investigate the nature of these component parts; (2) a study of 

 the antigenic activity of simple substances of known composition; (3) deliberate at- 

 tempts to alter the molecules of materials known to be good antigens in order to 

 study the effect of such changes upon their activity and specificity. 



THE FRACTIONATION OF COMPLEX ANTIGENS 



Most studies of this nature have been limited to the various proteins present in 

 antigenic materials. This has been due, no doubt, to the generally accepted idea that 

 only proteins can be antigenic. Nolf,' in 1900, first attacked the problem of separating 



• Nolf, P.: Ann. de Vlnst. Pasteur, 14, 297. igoo. 



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