In this case the cells introduced, or more properly, some substances within the cells, act as stimuli to the body cells of the animal injected to cause them to produce more of the specific cell receptors which respond to the stimulus. The substance within the introduced cell which acts as a stimulus (_antigen_) to the body cells is called an “_agglutinogen_.” That “agglutinogen” is present in the cell has been shown by injecting animals experimentally with extracts of cells (bacterial and other cells) and the blood serum of the animal injected showed the presence of agglutinin for the given cell. It will be noticed that the receptors which become the free agglutinins have at least _two functions_, hence at least _two chemical groups_. They must combine with the foreign cells and also bring about their clumping together, their agglutination. Hence it can be stated technically that an agglutinin possesses a _haptophore group_ and an _agglutinating group_. It is probable that the agglutination, the clumping, is a secondary phenomenon depending on the presence of certain salts and that the agglutinin acts on its antigen as an enzyme, possibly a “splitting” enzyme. This is analogous to what occurs in the curdling of milk by rennet and in the coagulation of blood. This probability is substantiated by the fact that suspensions of bacteria may be “agglutinated” by appropriate strengths of various acids. The formation of agglutinin in the body for different bacteria does not as yet appear to be of any special significance in protecting the animal from the organism, since the bacteria are not killed, even though they are rendered non-motile, if of the class provided with flagella, and are clumped together. The fact that such bodies are formed, however, is of decided value in the diagnosis of disease, and also in the identification of unknown bacteria.