Active Immunity

Active immunity occurs as a part of the human immune response, which is activated when a pathogen such as a bacterium or virus invades the body. The body recognizes this pathogen as a foreign substance and promptly begins producing antibodies (also called immunoglobulins) and other infection-fighting cells whose responsibility it is to rid the body of this foreign substance. On first exposure to the pathogen, the immune response is relatively slow and is typically accompanied by signs and symptoms of disease. However, the immune system retains memory of this pathogen. If this same pathogen invades the body again, the immune response, including the increased production of pathogen-specific antibodies, occurs much more rapidly and generally prevents disease. This active natural immunity may be present for the remaining life of the individual. Natural immunity is genetically determined in specific populations or families. Some pathogens cannot infect certain species because the environment is not suitable (e.g., measles cannot reproduce in dogs; thus, dogs have a natural immunity to measles).

Acquired immunity occurs from exposure to an antigen or from passive injection of immunoglobulins. Active protection against disease may also be provoked by immunization. Vaccination involves the administration of a small amount of antigen, which although capable of stimulating an immune response does not typically produce disease. The antigen in vaccines may be produced in several ways. Traditional vaccines contain the whole or components of an inactivated (killed) microorganism. Other vaccines are attenuated viruses composed of live, attenuated (weakened) microorganisms. Toxoids are inactivated toxins, the harmful disease-causing substance produced by some microorganisms.

Some newer vaccines are called conjugate vaccines. Such vaccines require a protein or toxoid from an unrelated organism to link to the outer coat of the disease-causing micro­organism. This linkage creates a substance that can be recognized by the immature immune system of young infants. Haemophilus influenzae type B is an example of a conjugate vaccine.

Recombinant subunit vaccines involve the insertion of some of the genetic material (e.g., deoxyribonucleic acid [DNA]) of a pathogen into another cell or organism, where the antigen is then produced in massive quantities. These antigens are then used as a vaccine in place of the whole pathogen. Hepatitis B is an example of this type of vaccine.

An adjuvant, often an aluminum salt such as aluminum hydroxide, aluminum phosphate, or aluminum potassium sulfate, is a substance sometimes used in the production of a vaccine to increase the vaccine’s immunogenicity and to prolong the immune response. Adjuvants may also be added to a vaccine to reduce the amount of antigen needed to produce a dose of vaccine. Adjuvants have been used in the production of immunizations since the 1930s and are found in many U.S. childhood vaccines.

Regardless of the composition of the vaccine, each vaccine is designed to stimulate an immune response against a specific pathogen. Booster doses are sometimes required to maintain sufficient immunity. Because the immune system retains memory, a vaccinated individual who is later exposed to the actual pathogen mounts a rapid immune response, thus preventing disease. This active artificially acquired immunity is the focus of this chapter.

Passive Immunity

Passive immunity occurs when an individual receives antibodies against a particular pathogen from another source. Newborn infants naturally receive passive immunity via the transfer of maternal antibodies across the placenta. Passive immunity may also be acquired through the administration of antibodies pooled from several human or animal sources that have been exposed to disease-causing pathogens. Alternatively, antibodies may be produced using recombinant DNA technology.

Whether natural or acquired, passive immunity is transient, lasting no more than several weeks to a few months. The recipient does not mount his or her own immune response. However, passive immunity is important. It helps young infants who, because of their immature immune systems, are poorly equipped to protect themselves against disease. Acquired passive immunity is important when (1) time does not permit active vaccination alone, (2) the exposed individual is at high risk for complications of the disease, or (3) the individual suffers from an immune system deficiency that renders that person unable to produce an effective immune response.

Vaccine-Preventable Diseases