VACCINATION

Vaccination refers to a procedure in which the presence of an antigen stimulates the formation of antibodies. The antibodies act to protect the host from future exposure to the antigen. Vaccination is protective against infection without the need of suffering through a bout of a disease. In this artificial process an individual receives the antibody-stimulating compound either by injection or orally.

The technique of vaccination has been practiced since at least the early decades of the eighteenth century. Then, a common practice in Istanbul was to retrieve material from the surface sores of a smallpox sufferer and rub the material into a cut on another person. In most cases, the recipient was spared the ravages of smallpox. The technique was refined by Edward Jenner into a vaccine for cowpox in 1796.

Since Jenner's time, vaccines for a variety of bacterial and viral maladies have been developed. The material used for vaccination is one of four types. Some vaccines consist of living but weakened viruses. These are called attenuated vaccines. The weakened virus does not cause an infection but does illicit an immune response. An example of a vaccination with attenuated material is the measles, mumps, and rubella (MMR) vaccine. Secondly, vaccination can involve killed viruses or bacteria. The biological material must be killed such that the surface is not altered, in order to preserve the true antigenic nature of the immune response. Also, the vaccination utilizes agents, such as alum, that act to enhance the immune response to the killed target. Current thought is that such agents operate by "presenting" the antigen to the immune system in a more constant way. The immune system "sees" the target longer, and so can mount a more concerted response to it. A third type of vaccination involves an inactivated form of a toxin produced by the target bacterium. Examples of such so-called toxoid vaccines are the diphtheria and tetanus vaccines. Lastly, vaccination can also utilize a synthetic conjugate compound constructed from portions of two antigens. The Hib vaccine is an example of such a biosynthetic vaccine.

During an infant's first two years of life, a series of vaccinations is recommended to develop protection against a number of viral and bacterial diseases. These are hepatitis B, polio, measles, mumps, rubella (also called German measles), pertussis (also called whooping cough), diphtheriae, tetanus (lockjaw), Haemophilus influenzae type b, pneumococcal infections, and chickenpox. Typically, vaccination against a specific microorganism or groups of organisms is repeated three or more times at regularly scheduled intervals. For example, vaccination against diphtheria, tetanus, and pertussis is typically administered at two months of age, four months, six months, 15–18 months, and finally at four to six years of age.

Often, a single vaccination will not suffice to develop immunity to a given target antigen. For immunity to develop it usually takes several doses over several months or years. A series of vaccinations triggers a greater production of antibody by the immune system, and primes the antibody producing cells such that they retain the memory (a form of protein coding and antibody formation) of the stimulating antigen for along time. For some diseases, this memory can last for a lifetime following the vaccination schedule. For other diseases, such as tetanus, adults should be vaccinated every ten years in order to keep their body primed to fight the tetanus microorganism. This periodic vaccination is also referred to as a booster shot. The use of booster vaccinations produces a long lasting immunity.

Vaccination acts on the lymphocyte component of the immune system. Prior to vaccination there are a myriad of lymphocytes. Each one recognizes only a single protein or bit of the protein. No other lymphocyte recognizes the same site. When vaccination occurs, a lymphocyte will be presented with a recognizable protein target. The lymphocyte will be stimulated to divide and some of the daughter cells will begin to produce antibody to the protein target. With time, there will be many daughter lymphocytes and much antibody circulating in the body.

With the passage of more time, the antibody production ceases. But the lymphocytes that have been produced still retain the memory of the target protein. When the target is presented again to the lymphocytes, as happens in the second vaccination in a series, the many lymphocytes are stimulated to divide into daughter cells, which in turn form antibodies. Thus, the second time around, a great deal more antibody is produced. The antibody response also becomes highly specific for the target. For example, if the target is a virus that causes polio, then a subsequent entry of the virus into the body will trigger a highly specific and prompt immune response, which is designed to quell the invader.

Most vaccinations involve the injection of the immune stimulant. However, oral vaccination has also proven effective and beneficial. The most obvious example is the oral vaccine to polio devised by Albert Sabin. Oral vaccination is often limited by the passage of the vaccine through the highly acidic stomach. In the future it is hoped that the bundling of the vaccine in a protective casing will negate the damage caused by passage trough the stomach. Experiments using bags made out of lipid molecules (liposomes) have demonstrated both protection of the vaccine and the ability to tailor the liposome release of the vaccine.

The nature of vaccination, with the use of living or dead material that stimulates the immune system, holds the potential for side effects. For some vaccines, the side effects are minor. For example, a person may develop a slight ache and redness at the site of injection. In some very rare cases, however, more severe reactions can occur, such as convulsions and high fever. However, while there will always be a risk of an adverse reaction from any vaccination, the risk of developing disease is usually far greater than the probability of experiencing severe side effects.