Human defence systems and vaccination

Biology · Infection and response. 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

The body has two lines of defence. The first is non-specific: it tries to stop any pathogen from getting in. The skin, nose, trachea, bronchi and stomach all do this.

If a pathogen does get in, the immune system tries to destroy it. White blood cells do this in three ways: phagocytosis, making antibodies and making antitoxins. Each antibody fits only one antigen, so it works against one type of pathogen only.

A vaccine puts a small amount of a dead or inactive form of a pathogen into the body. White blood cells respond by making antibodies. If the same live pathogen enters later, they make the correct antibodies quickly, so the person does not become ill.

Rules to remember

Skin: a barrier that also produces antimicrobial secretions. Nose: hairs and mucus trap particles.
Trachea and bronchi: mucus traps pathogens and cilia sweep the mucus up to the throat.
Stomach: hydrochloric acid kills most pathogens in food and drink.
Phagocytosis: a white blood cell engulfs a pathogen and digests it.
Antibodies bind to the antigens on one type of pathogen and help to destroy it. Antitoxins neutralise the toxins made by bacteria.
A vaccine contains no antibodies and is not a medicine: it makes the body produce its own antibodies.
Herd immunity: when a large proportion of people are immune, the pathogen cannot spread easily, so unvaccinated people are also less likely to meet it.

Common mistake

Students mix up antibodies, antitoxins and antibiotics. Antibodies and antitoxins are made by white blood cells; antibiotics are medicines that kill bacteria.

Worked example

After a first infection with a pathogen, antibodies appear in a girl's blood after 10 days. Months later the same pathogen infects her again. Antibodies appear after 2 days and reach a higher level. Explain the difference.

First infection: her white blood cells had not met this antigen before, so making the right antibody took time.
After the infection, her white blood cells can recognise this antigen.
Second infection: they make the correct antibodies much faster and in larger amounts, so the pathogen is destroyed before she feels ill.

Answer: Her immune system responds faster and more strongly the second time. Vaccination works in the same way.

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Infection And Response — Worksheet

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