Table of Contents
Passive immunization is a way of protecting an organism by giving it finished, ready‑made components of the specific immune response—usually antibodies—rather than stimulating it to make its own. In contrast to active immunization, no immunological memory is formed; the effect is fast but temporary.
Principles of Passive Immunization
In passive immunization, specific immune products are transferred from a donor to a recipient:
- The recipient is not (or is only minimally) involved in producing the immune response.
- The donor has previously formed antibodies (or antibody‑like molecules) against a particular antigen.
- The transferred substances act immediately against the pathogen or toxin.
Typically, these products are:
- Antibodies (immunoglobulins), often purified from blood plasma.
- In special modern approaches, monoclonal antibodies produced in cell culture.
The key features:
- Rapid onset: Antibodies are present and effective almost immediately after administration.
- Short duration: As the foreign antibodies are broken down, the protection disappears; there is practically no long‑term immunological memory.
- No primary response in the recipient: The recipient’s B and T lymphocytes are not activated as they would be in active immunization.
Natural and Artificial Passive Immunization
Natural Passive Immunization
This occurs without medical intervention, through physiological transfer of antibodies:
- Via the placenta: In many mammals, including humans, maternal IgG antibodies cross the placenta and enter the fetal circulation before birth.
- The newborn is temporarily protected against pathogens to which the mother has immunity.
- The protection wanes as maternal antibodies are degraded in the infant’s body.
- Via breast milk: Colostrum and early breast milk contain high levels of IgA and other antibodies.
- These antibodies protect the infant’s mucous membranes (e.g., gut) from pathogens, especially during the first months of life.
Natural passive immunization is particularly important because:
- The infant’s own specific immune response is not yet fully developed.
- It bridges the vulnerable period before active immunizations and the child’s own immune memory are fully established.
Artificial Passive Immunization
Here, antibodies are intentionally administered as a medical measure:
- Source of antibodies:
- Human donors (e.g., convalescent plasma or pooled immunoglobulin).
- Immunized animals (e.g., horses) for certain antivenoms and antitoxins.
- Biotechnological production (monoclonal antibodies).
- Forms:
- Specific immune globulins: Concentrated antibodies against a particular pathogen or toxin (e.g., hepatitis B immune globulin).
- Normal immune globulins: Mixed antibodies from many donors, providing broad, nonspecific support (e.g., in some immune deficiencies).
- Antitoxins and antivenoms: Antibodies directed against toxins (e.g., tetanus antitoxin, snake antivenom).
Artificial passive immunization is always time‑limited in its effect, but can be life‑saving in acute situations.
Mechanisms of Action
The transferred antibodies can:
- Neutralize toxins and viruses
- Bind directly to toxins (e.g., tetanus toxin, snake venom components), preventing them from interacting with target cells.
- Bind virus particles, blocking entry into host cells.
- Opsonize pathogens
- Coat bacteria or other microbes, making them easier for phagocytes to recognize and ingest.
- Activate complement (depending on antibody type)
- Promote complement‑mediated lysis of pathogens or enhance phagocytosis.
- Block pathogen binding sites
- Occupy structures on microbes or host cells needed for attachment, thus preventing infection or spread.
These mechanisms are identical to the functions of antibodies formed during a normal immune response; the difference lies only in the origin of the antibodies.
Indications and Typical Use Cases
Passive immunization is indicated when:
- Immediate protection is required (no time for active immunization to take effect).
- A high‑risk exposure has already occurred (post‑exposure).
- The patient is unable to mount a sufficient immune response, for example due to certain immunodeficiencies.
Examples of situations where passive immunization is used:
- Tetanus risk after injury
- If a person with uncertain or incomplete vaccination status suffers a deep or contaminated wound, tetanus immunoglobulin can be administered.
- Frequently combined with active immunization (see below).
- Rabies exposure
- After bites by animals suspected of carrying rabies, rabies immune globulin is given as quickly as possible around the wound and intramuscularly.
- Usually combined with rabies vaccination (active immunization).
- Hepatitis B exposure
- Newborns of mothers with active hepatitis B infection receive hepatitis B immune globulin soon after birth, often together with vaccination.
- Also used after needle‑stick injuries or sexual exposure in unvaccinated persons.
- Certain viral diseases
- Specific immunoglobulins can be used in special cases (e.g., varicella‑zoster immunoglobulin for particularly at‑risk individuals after exposure).
- Toxins and venoms
- Antivenoms for snake, scorpion, or spider bites.
- Antitoxins for botulism or diphtheria.
- Immune deficiencies
- Regular infusions of pooled immunoglobulins to support patients with antibody production defects.
In all these cases, the goal is immediate neutralization or mitigation of the pathogen or toxin to prevent disease or reduce its severity.
Duration of Protection and Lack of Memory
Passive immunity is temporary:
- Antibodies are proteins and are gradually broken down in the body.
- Typical half‑life of IgG in humans is on the order of weeks.
- As the antibody concentration declines, so does protective effect.
Crucially:
- No memory cells (B or T lymphocytes with long‑term memory) are generated in the recipient.
- Once the transferred antibodies are gone, the immune system is no better “prepared” than before.
Therefore, passive immunization:
- Does not replace full vaccination schedules.
- Is used as an acute measure or as a bridge until active immunization becomes effective.
Combined Passive–Active Immunization
In some situations, passive and active immunization are used together:
- Goal:
- Immediate protection through passive immunization.
- Long‑term protection through active immunization.
- Typical example:
- A person at risk for tetanus who is not fully vaccinated:
- Receives tetanus immune globulin (passive) for immediate neutralization of any possible toxin.
- Simultaneously receives tetanus vaccine (active) so that their own immune system can build memory.
Important practical aspects:
- The antibodies from passive immunization can, in high amounts, partially neutralize the vaccine antigen.
- The administration of immunoglobulin and vaccine is therefore often done at different injection sites and possibly with adjusted timing, depending on the preparation.
Risks and Side Effects
While generally useful and often life‑saving, passive immunization carries specific risks:
- Allergic reactions
- Especially when antibodies originate from animals (e.g., horse serum) rather than humans.
- Can range from local reactions to severe anaphylactic shock.
- Serum sickness
- A delayed immune reaction against the foreign proteins in the serum.
- Symptoms can include fever, skin rashes, joint pain, and swollen lymph nodes.
- Interference with active vaccination
- Transferred antibodies can neutralize live attenuated vaccines or other vaccine antigens if administered too closely in time.
- Vaccination schedules and timing must be carefully coordinated.
- Transmission risks (for human blood products)
- Modern processing and testing greatly reduce the risk of transmitting blood‑borne pathogens, but the potential exists and must be managed.
For these reasons:
- Human-derived immunoglobulins are preferred when possible.
- Patients must be monitored after administration, especially after animal-derived antivenoms or antitoxins.
Advantages and Limitations at a Glance
Advantages:
- Very rapid protective effect.
- Can be life‑saving in acute exposure to dangerous pathogens or toxins.
- Useful in individuals with impaired immune systems.
Limitations:
- Protection is transient; no immunological memory.
- Production is complex and costly (needs donors or biotechnological facilities).
- Risk of allergic reactions and serum sickness, particularly with non‑human antibodies.
- Possible interference with certain vaccinations.
Passive immunization is thus best understood as a targeted emergency tool and as supportive therapy, ideally combined with active immunization strategies that provide lasting protection.