Active and Passive Immunity

The body’s immune system can be divided into two broad types based on how immunity is acquired: active immunity and passive immunity. The distinction lies in whether the body itself produces the antibodies or receives them ready-made from an external source.

Active Immunity

When a host is exposed to antigens — which may be living microbes, dead microbes, or other foreign proteins — the host’s own immune system responds by producing antibodies. This is called active immunity.

The key feature is that the body does the work itself. Because the immune system must first recognise the antigen, activate B-cells, and then manufacture specific antibodies, active immunity is slow — it takes time to reach its full effective response.

Active immunity can arise in two ways:

• Natural infection — when infectious organisms gain entry into the body during a normal infection, the immune system mounts a response and produces antibodies.
• Immunisation (vaccination) — when microbes are deliberately injected (in a killed or weakened form) to trigger antibody production without causing the actual disease.

In both cases, the immunity that develops is long-lasting because memory cells are formed.

Passive Immunity

When ready-made antibodies are directly given to the body to protect against foreign agents, it is called passive immunity. Here, the host does not produce any antibodies; instead, antibodies from another source are transferred in.

Passive immunity acts immediately because there is no lag period for antibody synthesis. However, it is temporary — no memory cells are formed, so protection lasts only as long as the donated antibodies remain in the body.

The textbook gives two important examples of naturally occurring passive immunity:

• Colostrum — the yellowish fluid secreted by the mother during the first few days after childbirth (the initial days of lactation). Colostrum is rich in antibodies, particularly IgA, which protect the newborn infant from infections. This is why mother’s milk is considered essential for the newborn. …