254 INFECTION AND IMMUNITY 



The substances possessing this power have been conveniently named 

 antigen or antibody -producers by German writers. The term antigen 

 though etymologically wrong, nevertheless is convenient and has 

 crept into general usage. It signifies simply a substance which can 

 stimulate the production or formation of an antibody. Such sub- 

 stances, so far as is known, belong to the group of proteins and are 

 derivatives of animal or plant tissues. Being proteins, all antigens 

 are colloids. Recently, however, some crystalloidal substances have 

 been described as possessing antigenic properties. 



