552 THE RELATION OF COLLOIDS AND LIPOIDS TO IMMUNITY 



such as lecithin and the fatty acids, upon the phenomenon of hemolysis, 

 are closely related to, or to be explained by, the action of organic colloidal 

 solutions. 



THE COMPLEMENT-FIXATION TEST AS A COLLOIDAL REACTION 

 Many observations support the view that complement fixation by a 

 specific antigen and its antibody is really complement absorption by a 

 precipitate that forms when antigen and antibody are mixed. As 

 previously stated, all antigens are protein in character. While there is 

 some evidence to show that lipoids, and even carbohydrates, may act as 

 antigens, there is no doubt but that the chief antigenic principles of any 

 antigen are of protein structure; hence when mixed with an immune 

 serum containing specific antibodies, it is believed that an invisible 

 precipitate is formed that absorbs the complement. With serum anti- 

 gens the quantity of protein is so large that a precipitate can readily be 

 seen (the precipitin test). A serum antigen and its antibody may, 

 however, be so highly diluted that, when mixed, a precipitate is not 

 visible, although complement may be fixed (complement-fixation test 

 for the differentiation of proteins). Moreschi and Gay have contended 

 for many years that complements may become entangled and absorbed 

 in such precipitates. Reasoning on the basis of the colloidal theory, 

 it is possible that transition compounds of very diverse nature are 

 formed when antigen, antibody, and a complement are mixed. This 

 view on the action of complements and anti-complements is supported 

 by numerous investigators who have examined the question from the 

 standpoint of colloidal reactions. Thus in a complement-fixation test a 

 mixture of antigen, antibody, and a complement in definite proportions 

 results in the formation of new compounds of opposite electric charge, 

 which tend to aggregate in masses (although these may be so small as 

 to be invisible) and reduce their surface tension in just the same manner 

 as agglutination and precipitation are brought about after the colloidal 

 theories. When corpuscles and hemolytic antibody are subsequently 

 added, hemolysis does not occur because free complement is absent. 



A process similar to complement absorption by a specific antigen 

 and its antibody is the Wassermann reaction. According to the col- 

 loidal theories, this reaction may be explained as due to the formation 

 of an invisible precipitate by interaction between some substance in 

 *the serum of a luetic person (probably in the nature of an altered globu- 

 lin), complement, and lipoidal substances contained in an alcoholic or 



