Fighting Viruses: What Is Passive Immunization?

Antibodies are proteins that bind to and help attack pathogens, such as bacteria and viruses. They are a key component of the human immune system.

Typically, antibodies are produced by white blood cells in response to infection. But scientists have also developed ways to use antibody treatment to prevent disease. Vaccines, for example, trigger the production of antibodies in order to "teach" the immune system to fight future illness. Passive immunization provides similar, typically temporary, protection by "donating" antibodies, derived from humans or animals, to a vulnerable individual.

Passive immunity can develop naturally, such as when a mother's antibodies are transferred to a growing fetus or nursing infant. It also can occur artificially when antibodies that protect against specific pathogens are transferred, often via blood or plasma transfusion. Antibodies obtained from a source such as blood will include a mixture of variations, which arise as part of a natural immune response. These are called polyclonal antibodies, and they target different areas of an infectious agent. Monoclonal antibodies can be made in the laboratory and will all have the same target.

Several products have been approved by the U.S. Food and Drug Administration (FDA) for passive immunization and immunotherapy, including antibodies against botulism, diphtheria, hepatitis A, hepatitis B, measles, rabies, Kawasaki disease, and tetanus.

What are the advantages and disadvantages of passive immunization and immunotherapy?

Passive immunization and immunotherapy are particularly helpful for individuals who are immunocompromised and therefore cannot mount their own immune response. Passive immunization also works quickly, providing protection within hours or days, whereas vaccines can take weeks or months if boosting (more than one injection) is required. Unlike vaccination, however, passive immunization and immunotherapy do not result in immunological memory. Also, the transfer of antibodies from animals or other people can cause side effects including severe allergic reactions.

Can passive immunization and immunotherapy be used to treat or prevent COVID-19?

Convalescent plasma, the blood plasma of people who have recovered from COVID-19, and monoclonal antibodies have been used as a treatment for infected or exposed patients during the pandemic. Scientists and regulators continue to monitor the effectiveness of these treatments and make recommendations for health care providers as new variants of the virus emerge.

Investigators at Caltech contributed to the research behind antibody therapies by capturing the first-ever images of antibodies latching onto a key protein on the SARS-CoV-2 virus.

Read more about how vaccines provide immunity on the Caltech Science Exchange >