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Pathology => Microbiology => Immunization
Immunization
INTRODUCTION
Immunization, also called vaccination or inoculation, a method of stimulating resistance in the human body to specific diseases using microorganisms-bacteria or viruses-that have been modified or killed. These treated microorganisms do not cause the disease, but rather trigger the body's immune system to build a defense mechanism that continuously guards against the disease. If a person immunized against a particular disease later comes into contact with the disease-causing agent, the immune system is immediately able to respond defensively.
Immunization has dramatically reduced the incidence of a number of deadly diseases. For example, a worldwide vaccination program resulted in the global eradication of smallpox in 1980, and in most developed countries immunization has essentially eliminated diphtheria, poliomyelitis, and neonatal tetanus. The number of cases of Hemophilus influenzae type b meningitis in the United States has dropped 95 percent among infants and children since 1988, when the vaccine for that disease was first introduced. In the United States, more than 97 percent of children are immunized by the time they reach school age. In an attempt to continue these global successes, the World Health Organization (WHO) has set the year 2000 as a target date for the immunization of all children.
ACTIVE AND PASSIVE IMMUNIZATION
Active immunization involves injection of all or part of a disease-causing microorganism or a modified product of that microorganism into the body to make the immune system respond defensively. The activating components of the vaccination are antigens, substances that the immune system recognizes as foreign. In response to the antigen, the immune system develops either antibodies or white blood cells called T lymphocytes, which are special attacker cells. Immunization mimics real infection but presents little or no risk to the recipient. Some immunizing agents provide complete protection against a disease for life. Other agents provide partial protection, meaning that the immunized person can contract the disease, but in a less severe form. Some immunizing agents require repeated inoculations-or booster shots-at specific intervals. Tetanus shots, for example, are recommended every ten years throughout life.
A person can be actively immunized by receiving an injection of dead organisms that are no longer capable of causing disease but that still contain antigens. This type of vaccination is used to protect against bacterial diseases such as typhoid fever, whooping cough, and diphtheria. Active immunization can also be carried out using bacterial toxins that have been treated with chemicals so that they are no longer toxic, even though their antigens are still intact. This procedure uses the toxins produced by the bacteria rather than the organism itself and is used in vaccinating against tetanus, botulism, and similar toxic diseases. Finally, a person can be injected with live organisms that have been attenuated-that is, changed so that they do not cause disease. This procedure is used to protect against poliomyelitis, yellow fever, measles, smallpox, and many other viral diseases.
While active immunization confers long-lasting immunity, passive immunization provides temporary immunity and is performed without injecting any antigen. Passive immunization is accomplished by injecting antibodies obtained from the blood of an actively immunized human being or animal. The antibodies last for two to three weeks, and during that time the person is protected against the disease. Although short-lived, passive immunization provides immediate protection, unlike active immunization, which can take weeks to develop. Consequently, passive immunization can be life-saving when a person has been infected with a deadly organism.
Occasionally there are complications associated with passive immunization. Diseases such as botulism and rabies once posed a particular problem. Immune globulin (antibody-containing plasma) for these diseases was once derived from the blood serum of horses. Although this animal material was specially treated before administration to humans, serious allergic reactions were common. Today, human-derived immune globulin is more widely available and the risk of side effects is reduced.
There are more than 20 vaccines licensed in the United States. The American Academy of Pediatrics and the U.S. Public Health Service recommend a series of immunizations beginning at birth. The initial series for children is complete by the time they reach the age of two, but booster vaccines are required for certain diseases such as diphtheria and tetanus in order to maintain adequate protection. When new vaccines are introduced, it is uncertain how long full protection will last. Recently, for example, it was discovered that a single injection of measles vaccine, first licensed in 1963 and administered to children at the age of 15 months, did not confer protection through adolescence and young adulthood. As a result, in the 1980s a series of measles epidemics occurred on college campuses throughout the United States among students who had been vaccinated as infants. To forestall future epidemics, health authorities now recommend that a booster dose of measles vaccine be administered at 4 to 6 or 11 to 12 years.
Although modern immunizing agents generally are considered safe and effective, risks associated with the use of vaccines can vary from insignificant (fever and soreness at the site of injection) to life threatening. The goal in vaccine development is to achieve the highest degree of protection with the lowest rate of side effects. Today's genetic engineering technology makes it possible to prepare safer and highly effective genetically altered vaccines.
Not only children but also adults can benefit from immunization. Many adults in the United States are not sufficiently protected against tetanus, diphtheria, measles, mumps, and German measles. Health authorities recommend that most adults 65 years of age and older, and those with respiratory illnesses be immunized against influenza (yearly) and pneumococcus (once), which causes pneumonia.
HISTORY OF IMMUNIZATION
The use of immunization to prevent disease predated the knowledge of both infection and immunology. In China in approximately 600 BC, smallpox material was inoculated through the nostrils. Inoculation of healthy people with a tiny amount of material from smallpox sores was first attempted in England in 1718 and later in America. Those who survived the inoculation became immune to smallpox. American statesman Thomas Jefferson traveled from his home in Virginia to Philadelphia, Pennsylvania to undergo this risky procedure.
A significant breakthrough came in 1796 when British physician Edward Jenner discovered that he could immunize patients against smallpox by inoculating them with material from cowpox sores. Cowpox is a far milder disease that, unlike smallpox, carries little risk of death or disfigurement. Jenner inserted matter from cowpox sores into cuts he made on the arm of a healthy eight-year-old boy. The boy caught cowpox. However, when Jenner exposed the boy to smallpox eight weeks later, the child did not contract the disease. The vaccination with cowpox had made him immune to the smallpox virus. Today we know that the cowpox virus antigens are so similar to those of the smallpox virus that they trigger the body's defenses against both diseases.
In 1885, Louis Pasteur created the first successful vaccine against rabies for a young boy who had been bitten 14 times by a rabid dog. Over the course of ten days, Pasteur injected progressively more virulent rabies organisms into the boy, causing the boy to develop immunity in time to avert death from this disease.
Another major milestone in the use of vaccination to prevent disease occurred with the efforts of two American physician-researchers. In 1954 Jonas Salk introduced an injectable vaccine containing an inactivated virus to counter the epidemic of poliomyelitis. Subsequently, Albert Sabin made great strides in the fight against this paralyzing disease by developing an oral vaccine containing a live weakened virus. Since the introduction of the Sabin vaccine in 1961, polio has been nearly eliminated in many parts of the world.
As more vaccines are developed, a new generation of combined vaccines are becoming available that will allow physicians to administer a single shot for multiple diseases. Work is also under way to develop additional orally administered vaccines and vaccines for sexually transmitted diseases. Possible future vaccines may include, for example, one that would temporarily prevent pregnancy. Such a vaccine would still operate by stimulating the immune system to recognize and attack antigens, but in this case the antigens would be those of the hormones that are necessary for pregnancy.
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