Learning Objective
Differentiate between passive and active immunity in terms of mechanism, onset, duration, examples, and clinical applications.
Passive vs Active Immunity
| Feature | Passive Immunity | Active Immunity |
|---|---|---|
| Means of Acquisition | Receipt of pre-formed antibodies | Exposure to exogenous antigens leads to an immune response |
| Onset | Rapid (immediate protection) | Slow (days–weeks for antibody production) |
| Duration | Short-term, limited by antibody half-life (≈3 weeks) | Long-lasting, due to the development of memory B and T cells |
| Examples | • Maternal IgG crossing the placenta • IgA in breast milk • Antitoxins • Humanized monoclonal antibodies |
• Natural infection • Vaccines (live, inactivated, subunit) • Toxoid vaccines |
| Key Notes | • IVIG and other immune-globulin formulations provide temporary but specific passive immunity • No memory formation |
• Active immunity generates immune memory for future protection |
| Clinical Applications | • Used when immediate protection is needed (e.g., tetanus antitoxin, rabies IG) | • Used for long-term prevention (routine immunization schedules) |
Combined Passive + Active Immunization
Used when immediate and long-term protection are both needed, such as:
- Hepatitis B exposure: HBIG + Hep B vaccine
- Rabies exposure: HRIG + Rabies vaccine
