Active and passive immunity

Active and passive immunity are two fundamental ways the body gains protection against diseases.

Active Immunity

Active immunity is the type of immunity you acquire when your immune system actively produces its own antibodies after being stimulated by an antigen.

• Mechanism: Your own lymphocytes (B-cells and T-cells) are activated by antigens, leading to an immune response. This involves clonal selection and clonal expansion, where specific B-cells and T-cells rapidly divide and differentiate. Plasma cells, derived from B-cells, then secrete large quantities of specific antibodies. Crucially, memory cells (memory B-cells and memory T-cells) are produced and persist in the body for a long time, often a lifetime.

• Time to Develop: It takes a while for this protection to develop, typically 1–2 weeks, as the body needs time to mount a primary immune response. During this primary response, symptoms of the disease may appear.

• Duration: Protection is long-term (or permanent) because memory cells provide immunological memory. Upon re-exposure to the same antigen, these memory cells enable a much quicker, stronger, and longer-lasting secondary immune response, often preventing symptoms from developing.

• Types of Active Immunity:

  • Natural Active Immunity: Acquired when you become immune after catching a disease.

  • Artificial Active Immunity (Vaccination): Acquired when you become immune after being given a vaccination containing a harmless dose of antigen. Vaccines contain antigens that stimulate memory cell production without causing the disease.

Passive Immunity

Passive immunity is the type of immunity you acquire from being given antibodies made by a different organism; your immune system does not produce any antibodies of its own.

• Mechanism: It involves the direct transfer of pre-formed antibodies. Since the recipient's immune system is not activated, no memory cells are produced.

• Time to Develop: Protection is immediate. This is crucial in situations where immediate defense is needed, such as against potentially fatal diseases like tetanus, before the body's natural primary response can occur.

• Duration: Protection is short-term because the transferred antibodies are eventually broken down and removed from circulation by phagocytes.

• Types of Passive Immunity:

  • Natural Passive Immunity: Occurs when a baby becomes immune due to antibodies received from its mother, either through the placenta during pregnancy or in breast milk (colostrum). This protects the infant during a time when its own immune system is not fully developed.

  • Artificial Passive Immunity: Occurs when you become immune after being injected with antibodies from someone else, such as antitoxins against tetanus toxin. This provides immediate, but temporary, protection.

Key Differences Between Active and Passive Immunity

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