How does a depleted immune system bring on cancer?

This is a really interesting one. It's something that's only started to become clear relatively recently - the role of the immune system in preventing cancer.

For example, patients with HIV can have a very depleted immune system. This leaves them vulnerable to infections like viruses that can cause cancer, like human papilloma virus (HPV) and Epstein-Barr Virus (EBV); so one of the reasons why HIV-infected individuals have higher cancer rates might be because they succumb to cancer-causing infections more frequently. Indeed, the rates of lymphoma, a type of blood cancer linked with EBV infection, are about 100-fold higher in HIV positive individuals.

Also, it's thought now that the immune system actively patrols your body, spotting dodgy-looking cells that are destined to become cancerous and getting rid of them. So it follows that a disease that disables the immune system - like HIV - could increase the likelihood of a cancer developing. Indeed, it's the question of how the immune system may be able to recognise some cancer cells is a hot topic in research now. And whether we can turn it into overdrive and use "immunotherapy" to kick-start the immune system into killing cancer cells in patients, that's a very active area of research.

Another thing that's quite interesting is that the whole role of the immune system first started to become clear, partly through studying people with HIV and also through studying patients who've had transplants. They take immuno-suppressing drugs and are also more likely to get certain types of cancers. That started to make a link.

Also, there have been some interesting observations in patients with melanoma: doctors have occasionally noticed that some people with melanoma just spontaneously get better. That's thought to be their immune system waking up, recognising and then attacking their cancer. Scientists are now working on ways to stimulate this process to happen in everyone. So it's a really active field of research studying how the immune system is involved in this and we'll probably, in the future, see a lot more coming out about it.