11.06 Active & Passive Immunity
Types of Immunity
- Immunity refers to the body’s ability to resist and defend against infectious agents.
- There are two main types of immunity: Active Immunity and Passive Immunity.
- Each type can be further categorized based on how immunity is acquired.
1. Active Immunity
Definition:
- Active immunity involves the body actively producing its own antibodies in response to an antigen.
Mechanism:
- Activation of B and T Cells: When exposed to pathogens (bacteria, viruses), antigens present on these pathogens trigger B cells to produce antibodies and T cells to perform various immune functions.
- Memory Cell Formation: After the initial response, memory B and T cells remain in the body, providing long-term protection against future exposures to the same antigen.
Types of Active Immunity:
- Natural Active Immunity:
- Acquisition: Through natural infection with a pathogen.
- Example: Recovering from chickenpox leads to immunity against future chickenpox infections.
- Artificial Active Immunity (Vaccination):
- Acquisition: Through vaccination, where antigens are introduced via injections or orally without causing the disease.
- Vaccines: Contain weakened or inactivated pathogens, or specific antigens, to stimulate the immune system.
- Benefits: Provides long-term immunity similar to that obtained through natural infection without the risk of severe disease.
- Example: Measles, Mumps, and Rubella (MMR) vaccine.
Key Features of Active Immunity:
- Antigen Encounter: Yes, the immune system is exposed to the antigen.
- Immune Response: Active production of antibodies and memory cells.
- Antibody Appearance: Typically within 1–2 weeks after exposure.
- Protection Duration: Long-term, often lifelong.
2. Passive Immunity
Definition:
- Passive immunity is acquired by receiving antibodies from an external source rather than producing them through the individual’s own immune response.
Mechanism:
- Direct Transfer of Antibodies: Provides immediate protection by introducing ready-made antibodies into the body.
- No Memory Cell Formation: Since the individual’s immune system is not actively involved, there is no long-term immunity.
Types of Passive Immunity:
- Natural Passive Immunity:
- Placental Transfer:
- Process: Maternal antibodies (IgG) cross the placenta during pregnancy.
- Protection: Provides the fetus with immunity against pathogens the mother is immune to.
- Colostrum (Breastfeeding):
- Components: First milk contains high levels of IgA antibodies.
- Protection: Shields the infant’s gut and bloodstream from infections during early life.
- Placental Transfer:
- Artificial Passive Immunity:
- Acquisition: Through the injection of antibodies (antitoxins) from an external source.
- Usage: Provides immediate but temporary protection against specific toxins or pathogens.
- Example:
- Tetanus Antitoxin: Administered to individuals exposed to tetanus toxin to neutralize its effects swiftly.
- Monoclonal Antibodies: Used in treatments for various diseases, including certain viral infections and cancers.
Key Features of Passive Immunity:
- Antigen Encounter: No direct exposure to the antigen by the individual.
- Immune Response: No activation of the individual’s immune system.
- Antibody Appearance: Immediate availability of antibodies.
- Protection Duration: Short-term, typically a few weeks to months.
Comparison Summary: Active vs. Passive Immunity
| Feature | Active Immunity | Passive Immunity |
|---|---|---|
| Antigen Encounter | Yes | No |
| Immune Response | Activation of B and T cells; memory cell formation | Direct introduction of antibodies |
| Antibody Appearance | After 1–2 weeks | Immediately |
| Protection Duration | Long-term (often lifelong) | Short-term (weeks to months) |
| Examples | Natural infection, Vaccination | Maternal antibodies, Antitoxin injections |
Key Terms
- Vaccine: A preparation containing antigens that stimulate the immune system to develop immunity without causing the disease.
- Antitoxins: Injected antibodies used in artificial passive immunity to neutralize specific toxins.
- Colostrum: The first form of milk produced by mammals, rich in IgA antibodies, providing passive immunity to the newborn.
Immune Response Timeline
- Maternal Antibodies:
- Timing: Highest levels just before birth.
- Decline: Gradually decrease as the infant’s own immune system starts to develop.
- Infant’s Antibody Production:
- Onset: Begins shortly after birth.
- Development: Increases as passive immunity from maternal antibodies wanes, leading to active immunity as the infant is exposed to pathogens.
Additional Notes
- Memory Cells: Crucial for the rapid and robust response upon re-exposure to the same antigen, forming the basis of immunological memory.