Cancer cells are us, so as young offenders, they look the same as everyone else and blend in with the crowd. This makes life very taxing for the immune police. How could you pick out ‘a criminal’ standing beside law-abiding members of society in an identity parade? Luckily the immune system is exceedingly vigilant. A blessing when even a slight change can make one-of-us look like one-of-them. This will raise the alarm and trigger the armed immune system to remove the threat.

Many cancers are caused by viruses. Hepatitis B can lead to liver cancer. The HTL virus can cause a form of leukaemia. When the virus attacks or enters a cell it hijacks RNA or DNA for its own use, but may leave a clue, a print from a viral size 9 trainer, forensic evidence that is sufficient to activate the immune system. But how does the immune system recognise ‘one-of-us’? What are the identity credentials that police check when deciding who represents a potential threat in the community and who doesn’t?

Our cells each have the same identity tag, a combination of molecules called the MHC, unique to each of us. MHC tags tell our immune system what is ‘us’. They need to be closely matched when receiving a tissue transplant from someone else. Our immune system will only ‘see’ foreign antigens (infections from the outside world), if they sit in our cells alongside the same identity tag as the immune system itself bears. This is how we know the affected cell is one-of-us. If the police see an individual that physically looks like they fit in but they’re waving an offensive banner, the principle is the same whether it’s a normal cell with a virus or a cancer cell that’s not quite right – they will be arrested and removed. Recognizing foreign-plus-us is known as ‘antigen processing’.