THEORIES OF IMMUNITY 



153 



Two such antibodies are well known. In one we find that the toxo- 

 phore group of the antibody causes clumping or agglutination of its 

 antigen, or the agent that caused its production, and hence this antibody 

 is called an agglutinin. In typhoid fever, for example, the bacillus or 

 one of its more complex products causes the production of an antibody 

 of this nature, so that when the serum of a typhoid fever patient is 

 mixed with the bacilli, the latter lose their motility and form clumps or 

 agglutinated masses. This phenomenon was first observed by Gruber 

 and Durham, and was applied in a practical way to the diagnosis of 



FIG. 41. FORMATION OF AGGLUTININS AND PRECIPITINS. 



The central white area represents a molecule of a cell; the shaded portion repre- 

 sents the cell itself; the surrounding area represents the body-fluids about the cell. 



R, Receptor of the molecule (second order); R 2 , overproduction of receptors, 

 which are being cast off; A, a cast-off receptor which now constitutes the antibody; 

 A, A 2 , agglutinins in combination with the antigen (bacilli) . 



typhoid fever by Widal and Griinbaum. The second antibody of this 

 class, the precipitins, resemble the agglutinins quite closely (Fig. 41). 

 Kraus discovered that if a bouillon culture of the typhoid bacillus 

 is filtered through porcelain, and a few drops of serum from a typhoid 

 fever patient or from an animal immunized by injections of typhoid 

 bacilli are added to a small quantity of the bacilli-free filtrate, a faint 

 cloud will appear resembling in some respects that observed at the line 

 of contact between nitric acid and urine that contains a trace of albumin. 



