Antibodies are produced in the body in response to either infection with an organism or, through vaccination, the administration of a live or inactivated organism or its toxin by mouth or by injection. When given alive, the organisms are weakened, or attenuated, by some laboratory means so that they still stimulate antibodies but do not produce their characteristic disease. However stimulated, the antibody-producing cells of the body remain sensitized to the infectious agent and can respond to it again, pouring out more antibody. One attack of a disease, therefore, often renders a person immune to a second attack, providing the theoretical basis for active immunization by vaccines.

Antibody can be passed from one person to another, conferring protection on the antibody recipient. In such a case, however, the antibody has not been produced in the body of the second person, nor have his antibody-producing cells been stimulated. The antibody is then a foreign substance and is eventually eliminated from the body, and protection is short-lived. The most common form of this type of passive immunity is the transference of antibodies from a mother through the placenta to her unborn child. This is why a disease such as measles is uncommon in babies younger than one year. After that age, the infant has lost all of its maternal antibody and becomes susceptible to the disease unless protective measures, such as measles vaccination, are taken. Sometimes antibody is extracted in the form of immunoglobulin from blood taken from immune persons and is injected into susceptible persons to give them temporary protection against a disease, such as measles or hepatitis A.

Generally, active immunization is offered before the anticipated time of exposure to an infectious disease. When unvaccinated people are exposed to an infectious disease, two alternatives are available: active immunization may be initiated immediately in the expectation that immunity can be developed during the incubation period of the disease, or passive immunity can be provided for the interim period and then active immunization given at an appropriate time. The antigens (foreign substances in the body that stimulate the immune defense system) introduced in the process of active immunization can be live attenuated viruses or bacteria, killed microorganisms or inactivated toxins (toxoids), or purified cell wall products (polysaccharide capsules, protein antigens).

There are five basic requirements for an ideal vaccine. The agents used for immunization should not in themselves produce disease. The immunizing agent should induce long-lasting, ideally permanent, immunity. The agent used for immunization should not be transmissible to susceptible contacts of the person being vaccinated. The vaccine should be easy to produce, its potency easy to assess, and the antibody response to it measurable with common and inexpensive techniques. Finally, the agent in the vaccine should be free of contaminating substances. It is also recognized, however, that vaccine transmissibility can be helpful—e.g., in the case of live polio vaccine, which can be spread from vaccinated children to others who have not been vaccinated.

The route by which an antigen is administered frequently determines the type and duration of antibody response. For example, intramuscular injection of inactivated poliomyelitis virus (Salk vaccine) generates less production of serum antibody and induces only a temporary systemic immunity; it may not produce substantial local gastrointestinal immunity and, therefore, may not prevent the carrying of the virus in the gastrointestinal tract. Live, attenuated, oral poliomyelitis virus (Sabin vaccine) induces both local gastrointestinal and systemic antibody production; thus, immunization by mouth is preferred.

The schedule by which a vaccine is given depends upon the epidemiology of the naturally occurring disease, the duration of immunity that can be induced, the immunologic status of the host, and, in some cases, the availability of the patient. Measles, for example, is present in many communities and poses a potential threat to many children over 5 months of age. A substantial number of infants, however, are born with measles antibody from their mothers, and this maternal antibody interferes with an adequate antibody response until they are between 12 and 15 months of age. Generally, the immunization of infants after the age of 15 months benefits the community at large. In measles outbreaks, however, it may be advisable to alter this schedule and immunize all infants between 6 and 15 months of age.