560 IOWA DEPARTMENT OF AGRICULTURE 



consider immunity. Immunity may be of various types. We have, 

 for example, natural immunity, when individuals or species possess an 

 inherent quality which prevents them from contracting the disease to 

 which other individuals or species are susceptible. This immunity is 

 not brought about by any condition which occurs subsequent to birth. 

 We have acquired immunity when an attack of a certain infectious dis- 

 ease brings about a change which renders the individual immune to 

 further attacks of the same disease. Varieties of acquired immunity are 

 active and passive. The active immunity, which is usually of a lasting 

 nature, results from infection or intoxication and depends upon specific 

 reaction on the part of the tissue cells in response to the injury pro- 

 duced by the bacteria or their toxins. We have passive immunity when 

 an immune serum is injected and depends upon the introduction of im- 

 mune bodies rather than their production through an active process on 

 the part of the animal. 



Any one of these types of immunity may be relative or absolute, anti- 

 toxic, or anti-bacteria. If absolute, infection is impossible. There are 

 temporary conditions such as overwork, hunger or exposure when im- 

 munity is relative and infection is possible. Immunity is usually of the 

 relative type. 



The distinction between anti-bacterial and anti-toxic immunity is an 

 important one. The serum of an animal which has acquired immunity 

 to tetanus, neutralizes the soluble toxin, but does not necessarily injure 

 the tetanus bacillus itself. This is anti-toxic immtunity. On the other 

 hand where the serum of the animal is able to destroy or dissolve the 

 bacteria, as in typhoid, it is known as anti-bacterial immunity. 



ANTITOXIC IMMUNITY. 



Antitoxins are much more stable than toxins. The combination of 

 toxin and anti-toxin is direct and follows the laws of chemical com- 

 bination. The toxin is composed of two groups, a haptifore or combining 

 group, the other is the toxifore or poisonous group. The haptifore is 

 quite stable while the toxifore group is destroyed at 55 degrees C. or 

 decomposes on standing, but does not prevent the haptifore group from 

 uniting with a suitable antitoxin. It is only when the haptifore group 

 fits to the receptor of the body cell that the toxin can act. As a result 

 of this injury the body cells are stimulated and receptors are given off 

 in excess, and thrust into the circulation. These free cell receptors con- 

 stitute the antitoxin. If a toxin now enters the body, similar to the one 

 which leads to the production of antitoxin, the haptifore group of the 

 toxin will unite with the antitoxin and prevent the poison from damaging 

 the body cells. 



ANTIBACTERIAL IMMUNITY. 



In this, two constituents of the specific serum, are concerned in its 

 destructive powers, instead of one as in antitoxic immunity. One of 

 these is able to withstand heating to 55 degrees C. and is contained only 

 in the specific serum. The other is destroyed by heating to 55 degrees 

 C. and is contained in the serum of normal untreated animals as well 



