IMMUNITY 



can unite to themselves the poisonous substance, and the condition nec~ 

 essary in this instance is that a certain ferment-like substance called a 

 complement (or alexin) be present. These are protectors against infec- 

 tion. 



Complements can be demonstrated to exist in the laboratory. When 

 blood serum is placed in a test tube, the receptors do not 'unite with the 

 toxin if a very small amount of heat is applied to the serum proper. 

 Heat kills or paralyzes whatever it is which makes the union possible. 

 If, however, we add but a very small amount of unheated serum, the 

 union takes place almost immediately. Whatever it is that has been 

 destroyed by the heating and which permits or causes the receptor to 

 unite with the poisonous substance is called the complement. 



It is quite possible that certain cells of the body, or even all the 

 cells of some animals, may have no receptors at all for certain poisons, 

 and therefore, such cells and animals would have a natural immunity 

 toward those poisons. It is because the thrown-off receptor needs the 

 complement before it can anchor the poison that they have been called 

 amboceptors. 



The foreign body or substance is called the antigen, while the am- 

 boceptor produced by the action of injurious antigens is known as 

 the antibody. 



It is of great importance to know that the molecule which is the 

 amboceptor is decidedly specific. That is, an amboceptor will react only 

 to one specific foreign substance, so that antibodies formed in diphtheria, 

 for example, will not be the same as those formed in tetanus, nor will 

 they be able to assist in anchoring poisons produced in tetanus. 



Quite naturally, the rate and ability of metabolism in a cell will 

 determine how rapidly receptors are formed, and consequently will de- 

 termine how rapidly immunity can be brought about. This means, in 

 turn, that if the poisons can act more rapidly than the cells, the cells 

 as well as the possessor of those cells will succumb. 



Phagocytes (white blood cells which devour foreign substances) are 

 also subject to this same rate and ability of metabolism. Some phago- 

 cytes may devour a foreign body before the latter has time to bring- 

 about an injury. 



Some phagocytes may have no chemical substance within them 

 which can dissolve the invader, and so the invader may continue to live 

 even though engulfed by a phagocyte, or, the invader may even kill the 

 phagocyte. 



Then it must be remembered that in all parasitic organisms the 

 same conditions largely apply which apply in the host, so that just as 

 the host may strengthen his resistance so the parasite may strengthen 

 its virulence so as to overcome the increasing resistance of the host. 

 For example, capsules form about the bodies of the anthrax bacillus and 

 the pneumococcus, which makes them more resistant to any injurious 



