THE FACTS OF IMMUNITY 



Each of these topics could well be the subject of a lecture in 

 itself. Any briefer treatment will necessarily have to be 

 a rather dogmatic one, but I believe that it is possible in each 

 case to sort out the essentials needed to present a general 

 picture of the situation. 



I . The nature of ' antibody ' 



The first question to be considered is the physical nature of 

 an 'antibody'. A rabbit is inoculated with killed dysentery 

 bacilli on several occasions. A week or two later its serum 

 is found to agglutinate suspensions of the same Shigella to 

 high titre and we say it contains a specific agglutinin or 

 antibody. This system has been widely used in academic 

 immunology and can be taken as representative of the pro- 

 duction of 'classical circulating antibody'. 



It can readily be shown by appropriate physical fractiona- 

 tion that the functional activity of the serum is in the gamma 

 globulin fraction and with refined methods often in a relatively 

 sharp sub-fraction of the gamma globulins. This, however, 

 does not define the antibody as a uniform population of 

 gamma globulin molecules. As every serologist knows, 

 absorption of the serum with a series of more or less closely 

 related bacterial suspensions will leave behind antibody 

 solutions of quite distinct functional character. Using con- 

 ventional symbols, if the bacterial antigen has antigenic 

 determinants ABCDEF distributed over its surface, then the 

 reactive gamma globulin molecules in the serum cannot be 

 represented abcdef abcdef etc., but perhaps as «, b, c, d, e,f, 

 with some possibility that doubly reactive globulin molecules 

 such as ac, df may also be present. 



Antigenic determinants are probably rather small organic 

 configurations equivalent to not more than 2-4 amino-acid 

 or monosaccharide residues (Kabat, 1956). Each globulin 

 molecule carries one, two or three reactive sites capable of 

 binding to a particular determinant. What evidence there is 

 points to all the sites on one molecule having the same 



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