Types of Immunity: Active and Passive

Depending on how the antibodies protecting a person actually came to be present in their body, immunity is classified into two broad kinds: active immunity and passive immunity.

Active immunity is immunity the body builds for itself, by actively producing its own antibodies in response to an antigen. It can arise in two ways:

Naturally acquired active immunity develops when a person is infected by a pathogen and their own immune system responds by producing antibodies and memory cells against it — for example, a person who has recovered from chickenpox develops lasting active immunity to that virus.

Artificially acquired active immunity develops when a person is given a vaccine — a preparation containing a harmless or weakened form of an antigen (see section 7.6) — which stimulates the immune system to make its own antibodies and memory cells, without the person having to suffer the actual disease first.

Active immunity typically takes some days to develop fully, since the body has to manufacture the antibodies itself, but once established it is long-lasting, because memory cells persist and can be quickly reactivated on future exposure.

Passive immunity, by contrast, involves directly giving a person ready-made antibodies produced by someone — or something — else, rather than the person's own immune system making them. This too occurs in two ways:

Naturally acquired passive immunity occurs when antibodies pass from a mother to her child — across the placenta during pregnancy, and later through colostrum (the antibody-rich first breast milk) — giving the newborn temporary protection during the vulnerable early months of life.

Artificially acquired passive immunity is given medically, by injecting a person with antibodies (or antiserum) prepared in advance, usually because the threat is too immediate to wait for the body to mount its own response. Classic examples are the antitoxin given after a tetanus-prone injury, and antivenom given after a poisonous snakebite.