314 Microbes and You 



presented here merely to acquaint the reader with some of the 

 terminology and concepts of immunity as expressed through anti- 

 gens and antibodies. 



An antibody is defined in terms of antigens. That is, antibodies 

 are the substances produced as the consequence of specific stimu- 

 lation of certain body tissues by the agents we term antigens. 

 Antibodies are protective substances which can be found in the 

 blood and in other body fluids and tissues after a suitable lapse of 

 time following antigenic injection. The time period depends upon 

 the particular antigen and upon the serological response of the 

 animal injected. Many antigens will produce measurable quan- 

 tities of antibody in a week or less, with a maximum antibody level 

 (called titer) appearing in from ten to fifteen days after primary 

 antigen injection. 



Considerable controversy still exists relative to the nature of 

 antibodies, and as to whether one species of organism produces 

 manv different antibodies which we detect by different tests, or 

 whether there is but one antibody— the unitarian hypothesis. A 

 few of the recognized types of antibodies will be discussed. 



Agglutinins 



If whole cells are injected into an animal for which they are a 

 foreign substance, the blood serum can in time be shown to con- 

 tain a substance capable of causing the cells employed as antigen 

 to clump together, or to agglutinate. This antibody is called an 

 AGGLUTININ, and the antigenic cells are called agglutinogens 

 (make agglutinins). Cells in suspension tend to repel each other 

 because of physical phenomena including electrical charges, but 

 when the cells have been acted upon by agglutinins, something 

 happens at the cell surface which encourages cells to stick together, 

 or to agglutinate. 



This type of serological reaction is useful in the identification of 

 bacteria which are divisible according to their antigenic com- 

 ponents. When known antibodies react with given organisms, the 

 antigenic structure of those bacteria can be determined. Agglu- 



