The differences between active and passive immunity.

The idea

In active immunity the person's own immune system meets an antigen and responds. It can be natural, following infection, or artificial, following vaccination. The antigen triggers clonal selection, plasma cells secrete antibody and memory cells are produced, so protection takes days or weeks to develop but then lasts for years. In passive immunity the antibodies are made elsewhere and introduced ready-made: across the placenta, in colostrum and breast milk, or by injection of an antivenom or an anti-toxin. Protection is immediate because the antibodies are already present, but the person's own immune system has not responded to the antigen, so no clonal selection occurs and no memory cells are formed. Once the introduced antibodies are broken down, usually within weeks, protection is gone.

What earns the marks

• Active immunity involving exposure to antigen and the person's own production of antibody.
• Passive immunity involving antibodies introduced from outside, with the person's own immune system not producing them.
• Active immunity producing memory cells whereas passive immunity does not.
• The contrast in timing: passive protection is immediate but short-lived, active protection is slower to develop but long-lasting.
• A valid example on each side, such as vaccination and an anti-toxin injection or maternal antibodies.

Common mistakes and how to put them right

• Answering a 'contrast' question with facts about only one type.

  Put it right: The contrast command word requires both sides of each difference.

• Saying passive immunity lasts until the antigens are gone, when it ends as the introduced antibodies are broken down.

• Treating antibodies in breast milk as active immunity for the baby.

• Claiming passive immunity produces memory cells simply because antibodies are present.

• Assuming active immunity is always natural and passive immunity always artificial.

  Put it right: Both can be natural or artificial.

Exam tips

• Contrast questions need paired statements in one sentence: active produces memory cells whereas passive does not.
• Draft a two-column table in rough, source of antibody, speed, duration, memory cells, then turn each row into one comparative sentence.
• In data questions on anti-toxin treatment, name the group who should receive it and support the choice with a figure.