194 IMMUNOLOGY 



they react well with antibodies but either do not stimulate their 

 production or are very weak stimulators. 



Just why these antibodies are present in normal blood is not 

 definitely known. Some of them, like the isohemag£?lutinins, 

 heterohemagglutinins and lysins, are apparently normal physi- 

 ological products and it is possible that part of the antibodies foi' 

 infectious agents of variou.s kinds likewise are normal physiological 

 products. Sutliff and Davies (1937) studied nine infants over 

 a five-month period in an attempt to see whether there was any 

 relationship between the strain of pneumococcus in the naso- 

 pharynx and the type of antibody in the blood. They report that 

 no relationship could be noted although the pneumocoecidal action 

 of whole blood against Type II pneumococcus increased in three 

 infants and for T}T)e III pneumococcus in one infant. They 

 suggest the advisability of investigating the possible physiological 

 origin in future work. 



It is also possible that an undetermined amount of the so-called 

 normal antibodies for bacteria may owe their presence to unrecog- 

 nized infection Avilh tlie organism corresponding to the antibody. 

 The presence of the bacteria in the body due to unrecognized or 

 recognized infection or as a result of vaccination would represent 

 antigenic stimulation. Presumably this would result in antibody 

 production. 



Antigens. — As a ])relude to any further discussion of antibodies 

 it would seem desirable to recall some of the pertinent facts about 

 antigens that have either been mentioned or that appear in later 

 chapters. Both the colloidal state and solubility in the body fluids 

 seem to be properties of all true antigens. 



For a long time it was thought that only complete proteins could 

 stimulate specific antibodies and tlierefore l)e regarded as true 

 antigens. It is now quite generally agreed that the nitrogen con- 

 taining polysaccharide si)ecific for Type I pneumococcus and a 

 fetv other bacterial polysaccharides and perhaps a few lipoid- 

 carbohydrate complexes from cells and tissues can stimulate specific 

 antibodies when injected into certain animals. It was likewise 

 thought for a long time that antibodies would react only with 

 complete proteins, but Zinsser and Parker (1923) discovered non- 

 protein substances (haptens) in bacteria that reacted with bac- 



