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Understand the Role of the Immune System

Worksheet Overview

Did you know that the human body comes across hundreds and thousands of potential pathogens every day?


Image of bacteria, virus and fungi

The human body has loads of clever defences to protect itself from these invaders! These are called non-specific defences because they target any pathogen and aren't specific to a particular type. Let's look at these in more detail below.  


Image of bacteria



The First Line of Defence:



Physical Defences

The first line of defence is the skin. The skin is very tough and it pretty much covers the whole body. A pathogen has to get past the skin in order to get inside the body where conditions are great for replication. An ideal place for this would be something like a cut in the skin, for example. This is a physical barrier.

Another physical barrier is found in the nose. The hairs found in your nose help to trap any dust particles that may be a source of pathogens.


Image of ciliated cells



Mucus is another physical barrier. Mucus is produced by special cells found in the trachea and bronchi (the tubes that branch into the lungs).  It’s thick and sticky and perfect for trapping pathogens. The trachea is lined with special cells called ciliated cells. These contain tiny hairs which move backwards and forwards. This moves mucus out of the airways into your mouth (ready to be spat out or swallowed), a bit like a broom sweeping up a mess!

Chemical Defences

If a pathogen is sneaky enough to get past all of these physical barriers, our body uses chemical barriers to prevent infection. So what are these chemical barriers?


Image of eye



Enzymes in our eyes and saliva are an example of a chemical barrier. These enzymes are great at protecting us because they destroy any pathogen that tries to sneak in through our eyes or mouth.

Another example is the hydrochloric acid found in our stomach. This acid is very strong and can destroy pathogens that make their way to the stomach in food and drink. This is known as a chemical barrier.


So what happens if a pathogen gets past our first line of defence? This is where the immune system steps in -  our second line of defence.


The Second Line of Defence:

The immune system is made up of different types of cells. One type of cell is the white blood cell.


Image of antigens and antibody



Pathogens get into the body through bypassing the body's first line of defence. Pathogen cells have markers on their surface called antigens. Each pathogen has different antigens. White blood cells recognise these foreign antigens and will engulf and destroy these pathogens.

Other white blood cells make proteins called antibodies that fit the antigens of a pathogen like a puzzle. The antibodies stick to the pathogen’s antigens, allowing them to be clumped together.  This makes it easier for the pathogen to be detected and destroyed.

Some pathogens will produce toxins that make you feel ill. White blood cells will produce specific antitoxins that will neutralise the toxin.



If you get chickenpox when you’re young you can’t get it again. Your body has developed an immunity to the disease. 

After you have been infected with the chickenpox virus, some of the white blood cells that made antibodies against it stay in your blood. These are called memory cells.

If you’re infected again, the memory cells reproduce and make antibodies very quickly so your immune system responds much faster the second time.

You destroy the pathogen before it can make you ill.



In the following activity, you will investigate how the body protects itself from disease​. 

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