Active immunity
In active immunity the body makes its own antibodies after being exposed to an antigen. It develops more slowly, often taking one to two weeks to build up fully, but is long-lasting because memory cells are produced.
- Natural active — you catch a disease, recover, and your body has made antibodies and memory cells.
- Artificial active — a vaccine containing a weakened or dead pathogen (or its antigen) is given, so the body makes antibodies without becoming ill.
Passive immunity
In passive immunity the body receives ready-made antibodies from another source. Protection is immediate but short-lived, because no memory cells are formed and the antibodies are eventually broken down.
- Natural passive — antibodies pass from mother to baby across the placenta or in breast milk.
- Artificial passive — an injection of antibodies (antiserum) gives instant but temporary protection, for example after a snake bite.
Key idea
Active = you make antibodies (slow, long-lasting, has memory). Passive = you receive antibodies (fast, short-lived, no memory).
How vaccines work
A vaccine introduces harmless antigens that trigger the body's own antibody production and, crucially, memory cells. If the real pathogen invades later, the memory cells respond quickly, so the person does not fall ill. Booster doses may be given afterwards to strengthen the memory response, and widespread vaccination also protects a community through herd immunity.
Both active and passive immunity rely on antibodies, but the key difference is who makes them: in active immunity the person's own lymphocytes do, while in passive immunity the antibodies are simply borrowed from another source, which is why antiserum is used in emergencies when there is no time to wait.
Example
A baby is protected against some diseases in its first months by antibodies from breast milk (natural passive), then builds its own lasting protection through routine vaccinations (artificial active).