Immunity

Biology · Disease and immunity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Every pathogen has antigens on its surface with their own specific shapes. Lymphocytes make antibodies, which are proteins with a shape that is complementary to one antigen. Antibodies bind to the antigens. This destroys the pathogen directly or marks it so that phagocytes destroy it.

In active immunity the body makes its own antibodies, after an infection or after a vaccination. Memory cells are made too, so protection lasts a long time.

In passive immunity the antibodies come from another person: from the mother across the placenta or in breast milk. Protection starts at once but is short term, because no memory cells are made. HIV lowers the number of lymphocytes, so fewer antibodies can be made.

Rules to remember

Antibodies are proteins made by lymphocytes; each has a shape complementary to one specific antigen.
Active immunity: antibodies made in the body, after infection or vaccination; memory cells give long-term immunity.
Vaccine: weakened pathogens or their antigens, which stimulate lymphocytes to make antibodies and memory cells.
Passive immunity: ready-made antibodies from another individual (placenta, breast milk); short term; no memory cells.
If most of a population is vaccinated, the pathogen cannot spread easily, so unvaccinated people are protected too.
On a second contact with the same antigen, antibodies are made faster and in larger amounts.
HIV: fewer lymphocytes and reduced ability to produce antibodies, so the immune system is weakened.

Common mistake

Do not mix up antigens, antibodies and antibiotics. Antigens are on the pathogen, antibodies are proteins made by lymphocytes, and antibiotics are drugs. A vaccine contains antigens, not antibodies.

Worked example

A breast-fed baby is protected against a disease for its first few months, but must still be vaccinated against it later. Explain why.

The baby received ready-made antibodies from its mother across the placenta and in breast milk. This is passive immunity.
The baby's own lymphocytes were not stimulated, so no memory cells were made.
The mother's antibodies are broken down over a few months and are not replaced.
The vaccine gives antigens, so the baby's lymphocytes make antibodies and memory cells.

Answer: Passive immunity from the mother is short term. Vaccination gives active, long-term immunity because memory cells are made.

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