The immune system

The immune system is the body’s mechanism for protecting itself against the many harmful organisms which try to invade it, such as bacteria and viruses.

It is a highly complex network of cells, tissues and organs, including blood cells.

How does the immune system work?

When an organism gets inside your body (for example, by breathing in the common cold virus), white blood cells that form part of your immune system go to investigate. These white blood cells come in two groups: one group organises the other, which in turn produces the antibodies to fight the foreign organisms.

The cells that do the organising are called T-cells. Some of these have a protein on their surface, called CD4. We often refer to these as CD4 cells.

HIV and your immune system

HIV is dangerous because the virus destroys the very cells of your immune system that are supposed to be keeping you well: the CD4 cells.

the HIV life cycle

HIV attaches itself to a CD4 cell and enters it. It makes copies of itself inside the CD4 cell and then goes on to damage and destroy the cell. The new HIV viruses burst out of the CD4 cell and go off to find more cells to invade.

If the number of CD4 cells is reduced, the immune system has fewer cells to help it defend the body from other organisms. This means you are at greater risk of getting ill.

The immune system does try to fight HIV infection. It produces antibodies to do this. But they’re not very effective without the CD4 cells to organise them.

How does this affect your health?

Normally the body's immune system would fight off an infection, but HIV prevents the body’s immune system from working properly.

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