How does the immune system work?
When an organism gets inside your body (for example, 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.