BLOOD AND CIRCULATION 543 



that have developed in response to mumps viruses, for example, will not 

 combine with other antigens. It is believed that the specific configuration 

 of the antigen and antibody molecules resembles a lock and key. Only 

 antibodies that have developed in response to a given antigen can fit 

 on the surface of the antigen and react with it. 



The production of antibodies by certain of the body. cells continues, 

 perhaps for many years, after the patient has recovered. If a subsequent 

 invasion of the same type of antigen occurs during this period, anti- 

 bodies specific for it will already be present. The infected person does 

 not contract the disease and is said to be immune. The immunity that 

 is acquired as a result of having once had mumps, smallpox and certain 

 other infectious diseases lasts a very long time, generally for life. The 

 immunity to certain other diseases lasts for a much shorter time, and 

 after it is lost, one can get the disease again. 



One need not, however, get sick in order to develop an immunity 

 to many diseases. During the late eighteenth century, Edward Jenner 

 observed that milkmaids and others who handled the udders of cows 

 infected with cowpox never got smallpox. In 1796, he took a bit of the 

 material from the pustules of an infected cow and scratched it into 

 the skin of a person. Individuals so treated acquired a mild disease but 

 thereafter were immune to smallpox. Cowpox is caused by a virus known 

 as the vaccinia virus; smallpox by a different but related one known as 

 the variola virus. Vaccinia is not a serious disease in man, but it is similar 

 enough to variola so that antibodies that develop in response to it are 

 effective in combating variola. Jenner's experiments were the beginning 

 of the vaccination technique. Since then, many kinds of vaccination 

 have been developed. Usually a related and less virulent microorganism, 

 which could serve as the basis of a vaccine, is not available, but vaccines 

 can be produced by taking the actual disease organisms, rendering them 

 harmless by appropriate treatment, and injecting them. Although the 

 organisms are incapable of causing the disease, they are still capable of 

 inducing antibody formation. One of man's most recent triumphs over 

 disease has been the development by this method of a vaccine for polio- 

 myelitis. 



Immunities may be natural, be actively acquired, or be passively 

 acquired. All of us have a natural immunity to certain infectious diseases 

 that affect other organisms. Thus the virus for distemper, which is often 

 fatal to dogs, has no effect on man. It is probable that some of our 

 naturally occurring plasma proteins react with these invading antigens 

 before they can cause any trouble. Immunity that is acquired by an 

 exposure to the antigen, either by contracting the disease or by vac- 

 cination, is said to be active immunity, for the person exposed actively 

 produces the antibodies. A passive immunity can be acquired by inject- 

 ing serum containing antibodies that have been produced by another 

 individual or organism (antisera). A passive immunity lasts for only 

 a few weeks, so injections of antisera are used to help combat antigens 

 that have already invaded a patient rather than as a long-term pre- 

 ventive measure. 



