4 INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



motes phagocytosis, (h) Complement, if present, is generally taken up 

 when antibody and antigen combine ; thus, the occurrence of an other- 

 wise undetectable antibody-antigen reaction can sometimes be de- 

 duced from observation of complement fixation alone. The classical 

 Wassermann test for syphilis is based on this phenomenon, (i) The 

 combination of antibody and antigen may lead to the release of 

 histamine and other toxic substances from the tissues of the host, 

 as in anaphylaxis and allergy. 



All these effects of antibody, except probably those of class (i), 

 are thought to be beneficial to the host and to aid in resistance 

 to infection. All of them may, under suitable conditions, be utilized 

 in laboratory studies. 



However, although there is no doubt of the importance of antibodies 

 in immunity, they are by no means the whole story, and the natural, 

 more or less nonspecific mechanisms of resistance, such as im- 

 permeability of the skin and mucus membranes, and bactericidal 

 power of these body surfaces, the rise in body temperature which often 

 accompanies infection, the action of normal plasma components such 

 as complement and properdin, and the ingestion of invading micro- 

 organisms by the leukocytes (phagocytosis), are also important. In- 

 deed, of all the mechanisms of resistance, phagocytosis is probably 

 by far the most important. However, we shall here be concerned 

 with specific mechanisms of immunity and shall not further discuss 

 these other tools of resistance. 



Nature of Antibodies 



In view of the importance of antibodies in immunity and of their 

 theoretical interest as prime examples of specifically reacting bio- 

 logical substances, it is not surprising that many attempts have been 

 made to study their chemical nature. Thus far it has not been pos- 

 sible to ascertain by direct chemical analysis the structural basis for 

 the combining power and specificity of antibodies, because (a) it is 

 not easy to obtain large amounts of purified antibodies and (b) 

 protein chemistry is not far enough advanced for detailed knowledge 

 of the structure of any antibody molecule to be obtained. 



In spite of the difficulties, some preparations of purified antibody 

 have been studied. There has also been analytical work on antibody- 

 antigen compounds, which are more readily available in a relatively 



