We can raise our immunity against some infectious diseases by introducing into our bodies controlled doses of the organisms or their products that cause disease. These doses, known as vaccines, are most often introduced into the body by inoculation. After this, you are said to have been immunized or vaccinated, depending on the type of vaccine used. Inoculation is possible in several ways, including scarring of the skin, or injection into a muscle, vein, skin or other route. There are three main kinds of vaccine: 1. Organisms that have been weakened by growing them for many generations of the pathogen in artificial conditions (live attentuated organisms), such as the organisms that cause smallpox, polio, measles, rubella (German measles) and yellow fever. 2. Organisms that have been killed by heat or chemicals and are thus totally inactivated, such as those causing pertussis (whooping cough), typhus, influenza, measles and polio. (Measles and polio vaccines can be produced from organisms that are either live attentuated or killed.) 3. Toxins produced by organisms, such as those causing diphtheria and tetanus.

The type of vaccine given depends on which disease one needs immunization against. Once inoculated with a vaccine, the body responds actively by forming antibodies or antitoxins. These destroy the vaccine in the body, but the body is able to recognize these organisms or toxins and can subsequently respond rapidly if presented with them again. Antibodies are proteins synthesized and secreted by plasma cells in response to the presence of foreign proteins (antigens) and are responsible for destroying them. Antibodies, also known as immunoglobulins, are produced when the body is invaded by a bacterium or virus or when an organ is transplanted from another person. In the case of infection, the antibody response may not be powerful enough to ward off the disease. But if the ‘blueprint’ for the antibody response is present in the body, as is the case after immunization, then the disease can be defeated.

Discovery of vaccines

The English physician Edward Jenner (1749-1823) was one of the first to realize the importance of vaccination. Jenner noticed that milkmaids were resistant to smallpox. The cowpox virus, which causes a mild disease that infects the hands of milkers via the cow’s udders, closely resembles the smallpox virus. Cowpox produced antibodies in the milkmaids that provided protection against smallpox as well. Jenner started vaccinating people against smallpox by injecting them with the lymph fluid of cows that had suffered from cowpox.

The type of vaccine used nowadays determines the number of doses needed to acquire adequate immunity against a disease. Live attentuated organisms multiply in the host to produce a large number of antigens and, consequently, large quantities of antibodies. Hence, only a single dose is needed to induce long-lasting immunity against diseases such as polio and measles. Killed organisms or toxins require two or more doses to induce effective immunity and are used to produce TAB bracketa combined vaccine against typhoid and paratyphoid A and B fevers), pertussis and diphtheria vaccines.

Immunization programmes

Most developed countries offer free immunity against diphtheria, tetanus, whooping cough (pertussis), poliomyelitis, measles, German measles (rubella) and tuberculosis. This group of vaccines is used in routine immunizations in general medical and public health programmes. For people who travel to tropical countries, or workers in certain hazardous occupations, immunization is available against hepatitis B, typhoid, paratyphoid, plague, cholera, typhus, yellow fever and other diseases more rarely encountered. Every young child should have protection against diphtheria, tetanus and polio. In addition, a mumps and measles inoculation can be asked for. As they get older, girls should be immunized against rubella, because rubella in pregnancy could damage the foetus. Often a ‘triple’ vaccine of diphtheria, tetanus, and pertussis (DTP) is given in one injection, followed by two more doses at six- to eight-week intervals. Although there is a chance of serious side-effects in young children given the pertussis vaccine, the risk of catching whooping cough, which can be extremely debilitating, is thought by many to outweigh the vaccination risk.

Taking a broad view, immunization against a disease is only an intermediate stage in a process whose ultimate aim is to eradicate a disease completely. With worldwide vaccination against smallpox, for example, what was once regarded as the world’s most fatal disease has been virtually wiped out. Vaccines against influenza, however, a disease that may have serious consequences for the elderly, are of variable benefit because there are so many strains of the influenza virus. When an influenza outbreak occurs it is usually possible to produce a vaccine giving protection for six to nine months, but the same vaccine seems to have poorer protective qualities against the next outbreak.

Active and passive immunity

The type of immunity conferred by the methods described above is known as artificial active immunity, because it has been artificially induced and because the body’s own defence system is mobilized to produce antibodies. However, it is possible to achieve temporary protection against certain diseases by injecting someone with serum from an animal or person who has just recovered from the infection in question and who therefore possesses specific antibodies against the disease. This process is known as providing artificial passive immunity. This can be useful, for example, in the case of a measles outbreak. If a child has not yet been immunized, active immunization at this stage would be useless because the child would not create antibodies in response to the vaccine in time to ward off infection by the measles virus. But the injection of some serum concentrated from the blood of someone who has just had measles, or has just been immunized against it, provides the child with circulating antibodies that confer instant, but short-term, protection against measles for three or four weeks, until the circulating antibodies are used up. Artificial passive immunity can also be valuable in the treatment of tetanus, diphtheria, gas gangrene, snake bites and immunodeficient conditions. Passive immunity occurs naturally between a mother and her unborn child, because antibodies are transferred from the mother across the placenta to her