General Principles of Immunization

GENERAL PRINCIPLES OF IMMUNIZATION

Immunization is the most effective method to provide individual and community protec-tion against many epidemic diseases. Immunization can be active, with stimulation of the body’s immune mechanisms through administration of a vaccine, or passive, through ad-ministration of plasma or globulin containing preformed antibody to the agent desired. Active immunization with living attenuated organisms generally results in a subclinical or mild illness that duplicates to a limited extent the disease to be prevented. Live vaccines generally provide both local and durable humoral immunity. Killed or subunit vaccines such as influenza vaccine and tetanus toxoid provide immunogenicity without infectivity. They generally involve a larger amount of antigen than live vaccines and must be admin-istered parenterally with two or more spaced injections and subsequent boosters to elicit and maintain a satisfactory antibody level. Immunity usually develops more rapidly with live vaccines, but serious overt disease from the vaccine itself can occur in patients whose immune responses are suppressed. Live attenuated virus vaccines are generally con-traindicated in pregnancy because of the risk of infection and damage to developing fetus. Recent developments in molecular biology and protein chemistry have brought greater sophistication to the identification and purification of specific immunizing antigens and epitopes and to the preparation and purification of specific antibodies for passive protec-tion. Thus, immunization is being applied to a broader range of infections.

Prophylaxis or therapy of some infections can be accomplished or aided by passive immunization. This procedure involves administration of preformed antibody obtained from humans, derived from animals actively immunized to the agent, or produced by hy-bridoma techniques. Animal antisera induce immune responses to their globulins that re-sult in clearance of the passively transferred antibody within about 10 days and carry the risk of hypersensitivity reactions such as serum sickness and anaphylaxis. Human anti-bodies are less immunogenic and are detectable in the circulation for several weeks after administration. Two types of human antibody preparations are generally available. Im-mune serum globulin (gamma globulin) is the immunoglobulin G fraction of plasma from a large group of donors that contains antibody to many infectious agents. Hyperimmune globulins are purified antibody preparations from the blood of subjects with high titers of antibody to a specific disease that have resulted from natural exposure or immunization; hepatitis B immune globulin, rabies immune globulin, and human tetanus immune globu-lin are examples.