What is immunity and how does it work?

What is immunity?

The immune system is your body’s way of helping to protect you from infection. When your body is infected by viruses, bacteria or other infectious organisms (e.g. a fungus or parasite), it undergoes a process of fighting the infection and then healing itself.

As a result of this, the next time your body encounters the same organism, you will be 'immune' to this infection. This means that you are less likely to get the same disease again, or if you do, the infection will be less severe. This is the principle behind vaccination.

How does immunity work?

Whenever your body encounters a foreign organism, like bacteria or a virus, a complicated set of responses are set in motion. Your body has two sets of defensive mechanisms, one called ‘innate immunity’ and another called ‘adaptive immunity’.

Innate immunity

Innate immunity describes your body’s barriers to infection that are in-built (or innate). This includes:

  • your skin
  • the acid in your stomach
  • saliva
  • tears
  • mucus in your mouth and nose
  • cells in your blood stream that can destroy bacteria.

All of these systems are extremely important as a first line of defence to prevent you from becoming infected, and for getting rid of the infections that you do get.

These innate systems do not change with multiple exposures to the same infection; there is no ‘learned’ response no matter how many times your body is exposed to the same organism.

Adaptive immunity

Your body’s more complicated second line of defence is called adaptive immunity. By adapting to fight infections from particular bacteria or viruses, your body can become immune to infections caused by the same organism in the future. This adaptation by your body to prevent infection is the basis of immunisation.

Certain types of blood cells can learn from exposure to an infection. This means that the next time they encounter that infection they can remember it and mount a faster and stronger response.

For example:

  • antibodies are made by the body in response to an infecting organism. They can recognise specific types of viruses or bacteria. They work by attaching themselves to the organism, and preventing them from infecting your body.
  • macrophages are specialised blood cells that can directly attack and destroy an infecting organism, digesting them so they can't produce disease.
Vaccines trigger the adaptive immune system — by stimulating the body to make antibodies — so that it can prepare for a potential infection in the future.

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