Disease Transmission and Mans Resistance 313 



fetus during the gestation period, the fluid portions and the sokible 

 protein fractions are transferred to the developing fetus. Since 

 the fetus has done nothing to build up these antibodies, it is a 

 NATURAL, PASSIVE, ACQUIRED IMMUNITY. If autibodics built up in 

 other animals are injected into a person, this is artificial, passive, 



ACQUIRED IMMUNITY. 



ANTIGENS AND ANTIBODIES 



An ANTIGEN is any substance, usually protein or protein-like, 

 which when introduced into the body by some route other than 

 through the digestive tract, will induce the production of anti- 

 bodies which are capable of reacting specifically with the antigen 

 which caused their stimulation. It is the chemical structure of the 

 antigen which deteiTnines specificity, and the corresponding anti- 

 body might be looked upon as a lock and key type of relationship. 

 Individual keys rather than master keys are the rule in antigen- 

 antibody reactions. In order to serve as an antigen, the material 

 must be foreign to the animal into which it is being introduced. 

 Different portions of a single bacterial cell may function as in- 

 dividual antigens. The flagella, for example, are chemically dis- 

 tinct from the rest of the cell, and therefore the flagella will 

 stimulate production of specific antibodies which are distinguish- 

 able from the cellular (somatic) antibodies. 



Some materials while not antigenic in themselves will, when 

 linked with proteins, impart to the host protein a specificity differ- 

 ent from that of the underlying protein. These substances, some- 

 times carbohydrates and other times fats, are termed haptenes, or 

 partial antigens. The more than seventy types of pneumococci 

 are differentiated from each other on the basis of such carbo- 

 hydrate haptene fractions found in the capsule laver. The under- 

 lying pneumococci are very similar, if not identical with each other, 

 once the capsule has been stripped away. 



It is difficult to know how far to go with this rather complex 

 yet fascinating topic of antigens and antibodies. Volumes have 

 been written on the subject, but a few of the fundamentals will be 



