Protection from infectious disease is called immunity. There are two basic means of acquiring immunity—passive and active. Active immunity is produced by the person's own immune system. Passive immunity is produced by an animal or human and transferred to another human, usually by injection. The immune system is a complex system of cells that identify foreign substances called antigens. Antigens can be live or inactive. The immune system produces defense against antigens, which is known as the immune response. The immune response involves the production of proteins called antibodies and specific cells that facilitate the elimination of antigens.

Passive immunity is the transfer of antibody produced by one human or animal to another. The most common form of passive immunity is that which an infant receives from its mother. These antibodies will protect an infant from certain disease for up to a year. All passive immunity is temporary and disappears over time. The administration of immune globulin also provides passive immunity. Immune globulin is derived from the IgG antibody fraction from thousands of adult donors. Because it comes from many donors, it contains many different antibodies. It is usually used for post-exposure prevention of hepatitis A and measles.

Active immunity is the stimulation of the immune system to produce antigen-specific protection. Active immunity usually lasts for many years, often for a lifetime. One way to acquire active immunity is to have the natural disease. Usually, once a person recovers from an infectious disease, they will be immune to that disease for the rest of their lives. This is known as immunologic memory. Another way to produce active immunity is by vaccination. Vaccines interact with the immune system and often produce an immune response similar to that produced by the natural infection but without actually having the disease. Vaccines also produce immunologic memory.

There are two basic types of vaccines: live attenuated and inactivated. Live attenuated vaccines are produced by modifying disease-producing viruses or bacteria in a laboratory. The resulting vaccine organisms retains the ability to replicate and produce immunity but usually do not cause the illness. Inactivated vaccines are composed of either whole viruses or bacteria or fractions of them. Fractional vaccines are either protein-based or polysaccharide-based. Protein-based vaccines include toxoids which are inactivated bacterial toxins. They also include subunit or subvirion products. Most polysaccharide vaccines contain pure cell wall polysaccharide from bacteria. Conjugate polysaccharide vaccines link a protein to the polysaccharide to make a more potent vaccine. As a general rule, the more similar a vaccine is to the natural disease, the better the immune response is to the vaccine.

Live attenuated vaccines can be destroyed by heat and light. They must be handled and stored carefully. To produce the immune response, live attenuated vaccines must grow in the vaccinated person. Currently available live attenuated vaccines include measles, mumps, rubella, rotavirus, yellow fever, vaccinia, and varicella, which are viral diseases. Live bacterial vaccines include BCG and oral typhoid. Inactivated vaccines are not alive and cannot replicate. These vaccines cannot cause disease from the infection and cannot mutate to a form that can cause disease. Inactivated vaccines always require multiple doses. The first dose only "primes" the immune system. An immune response develops after the second or third dose. Currently available inactivated vaccines include influenza, polio, rabies, hepatitis A, pertussis, typhoid, cholera, plague, hepatitis B, Lyme disease, diphtheria, tetanus, pnuemococcus, meningococcus, and Haemophilus influenza type b.

Reference: Epidemiology and Prevention of Vaccine-Preventable Diseases, 5th edition, January, 1999, Centers for Disease Control and Prevention.

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