Our immune system is one of our bodies most powerful tools. Without it we would be victim to attacks from viruses, bacteria and fungai all the time. These micro organisms live everywhere, even on us!
The importance of the immune system is shown through a disease called severe combined immunodeficiency or SCID. Babies born with SCID often die before they are 1 unless they get a new immune system from a bone marrow transplant.
HIV also destroys our immune systems by killing off T cells. T cells are important for killing off tumours and cells infected with viruses. Patients with HIV are often more vulnerable to viral infections and contracting cancer as a result.
The immune system also keeps us safe from cancer. On average our immune system removes between 5-10 tumours a day inside us. Cancer only arises when particularly nasty tumours emerge that can hide from and switch off the immune system.