NATURAL AND ACQUIRKD IMMUNITY 121 



antigenic components of E. typhosa and Mudd, Pcttit, and Lack- 

 man and Morgan (1939) studying the antigenic components of 

 virulent streptococci. This work has been mentioned under viru- 

 lence in Chapter I. It will be recalled that virulence is associated 

 with capsule formation and that each kind of bacterial cell is made 

 up of a number of antigenic components, any one of which may give 

 rise to antibodies, depending perhaps upon its location in the cell. 



It ha.s been emphasized by Landsteiner, Nicollc, Zinsser and 

 others that there is a "mosaic" of antigens in a bacterial cell. 

 The position and the way the antigens are oriented relative to the 

 surface are important factors. The surface antigens may act as a 

 barrier to antigenic components beneath the surface. These are im- 

 portant concepts when one is considering the factors that determine 

 the immunizing value of a vaccine. It is known, e.g., that E. 

 typhosa can dissociate into smooth motile, smooth nonmotile, 

 rough motile and rough nonmotile variants. 



It was thought, until 1934, that the best typhoid vaccine should 

 consist of any smooth motile strain because it would contain 

 (somatic) and H (flagellar) antigens. Felix and Pitt, however, 

 discovered that an additional antigenic factor determining viru- 

 lence is important. They named this factor the "Vi" antigen.* It 

 is apparently a surface antigen and is destroyed by lieat and many 

 chemicals. More recently Mudd and his associates (1938, 1939) 

 have shown that virulent streptococci contain a partial heat labile 

 antigen obtained by physical means of disintegration. It is prob- 

 able that many and perhaps all virulent bacteria contain labile 

 antigens important in immunization. It is perhaps because of the 

 heat lability of these antigens and the ease with which they are 

 destroj^ed by most chemicals that killed bacteria are not as good 

 immunizing agents as virulent living ones. 



Apparently formaldehyde can be used to kill bacteria with only 

 partial destruction of the "Vi" antigen. This may be the reason 

 formalized vaccines have been reported as superior to heat killed 

 ones. 



Passive Immunity. — When a serum containing antibodies is in- 

 jected into a patient, the latter is being passively immunized. His 

 own tissue cells did not produce these antibodies. In other words, 



♦Topley and associates have isolated two carbohydrate-lipid complexes cor- 

 responding to O and Vi antigens respectively. Lancet 2.S2: 252, 193 7. 



