CHAPTER VIII. 
ANTIGENS AND THE TECHNIQUE OF SERUM REACTIONS. 
Nature of Antigens and Antibodies. 
Agglutinins and Precipitins. 
Lysins. 
Hemolysis and the Complement-fixation 
Reaction. 
Kahn Test in the Diagnosis of Syphilis. 
Aggressins. 
Opsonins, Tropins. Bacterial Vaccines. 
NATURE OF ANTIGENS AND ANTIBODIES. 
Those substances which cause specific antibody formation when 
they are introduced into the tissues or the body fluids of the host are 
called antigens. Their chemistry is as yet unknown/ but available 
evidence would indicate that they are for the most part proteins, 
colloidal in nature and highly organized chemically. Recent work 
indicates that complex substances apparently related to polysaccharides 
may play some part in immune processes.^ Degradation products of 
proteins, as albumosesand peptones, as well as carbohydrates and fats, 
are not ordinarily antigenic, that is, they do not lead to antibody 
formation when they are introduced into the animal body.^ The 
antigenic properties of lipoids are still a subject of controversy: lipoids 
appear to play a prominent part in certain types of immunological 
reactions,* but their ability to stimulate specific antibody formation 
cannot be regarded as proven at the present time. 
The function of antibodies as specific offensive weapons of the 
host against alien organisms or their products has long been recog- 
nized in bacteriology, and most important laboratory diagnostic 
methods have been elaborated through a study of the reactions between 
specific antigens and their respective antibodies. Antibodies are 
soluble and are found in various concentrations in blood serum derived 
from immunized animals. Many attempts have been made to deter- 
mine changes in the chemical composition or physical properties of 
immune sera from those of normal serum. Atkinson,^ Gibson and 
Banzhaf '^ and others have found that the sera of horses immunized to 
1 See Pick, Kolle, and Wassermann: Handb. d. path. Mikroorgan., 2 ed., Bd. I, 
p. 684, for discussion of the chemistry of antigens. Also Wells: Chemical Aspects of 
Immunity, 1925. 
2 See Heidclberger: Chem. Rev., 1927, 3, 403. 
' The injection of carbohydrates and fats may, however, lead to specific enzyme 
formation. See Rohmann, Antigene W^irkung der Kohlenhydrate, Deutsch. med. 
Wchnschr., 1914, 40, 204. 
4 W'arden: Jour. Infec. Dis., 1918, 23, 504; 1919, 24, 399. 
<• Jour. Exper. Med., 1901, 5, 67. 
6 Ibid., 1910, 12, 411. 
