Viruses, bacteria and cancer cells all cause disease in humans and animals. What words might you use to describe them? Virulent, invasive, destructive? Maybe. Evasive, smart, clever? Perhaps not. But many of the mechanisms of viruses, bacteria and cancer cells are just that. All three have evolved a myriad of ways to avoid detection by the immune system, and even smart techniques to “trick” it.
Viruses are tiny, pathogenic organisms which invade cells and replicate within them. Cells have mechanisms to “flag up” viral infection to the immune system – they display a special molecule called MHC1, which signals to the immune system that the cell is infected with a virus. Killer T cells can attach to the MHC1, and are activated to inject toxic granules into the infected cell, causing it to die. But this system is far from foolproof, and viruses have many clever tricks up their sleeve to prevent MHC1 being displayed on the cell surface. Viral proteins can stop the synthesis of MHC1, prevent its transport to the cell surface and even target it for degradation within the cell. If no MHC1 is displayed, then the immune system cannot detect the cell as infected, and the virus can continue to infect and replicate within human cells.
Bacteria are larger organisms than viruses, so act extracellularly rather than intracellularly, by releasing toxins which cause disease. The immune system deals with bacterial infection by sending specialised cells to engulf the bacteria – a process known as phagocytosis. And you guessed it, some bacteria have evolved techniques to avoid being phagocytosed. An example is Yersinia bacteria, which inject macrophages with effectors which prevent their phagocytic activity.
And what about cancer cells? These cells are perhaps the elite of immune system evasion. Cancer cells can release a range of chemicals which “dampen” the immune response, as well as secreting collagen to form a protective layer over them to “hide” from immune cells. Tumour cells do not express any receptors which would flag them up as dangerous to the immune system. In fact, if an immune cell such as a T cell does contact a tumour cell, the tumour cell can induce that T cell to become inactivated. Essentially, the tumour cells are imposters, tricking the immune system in to thinking they are normal body cells so that they are left alone to proliferate and spread to other areas of the body. This technique, whereby cancer cells pretend to be normal cells, has to be the ultimate in immune system evasion – why would a functioning immune system attack its own cells? Viruses, bacteria and cancer cells have very different mechanisms of action and disease pathology, yet they share the common characteristic of smart evasion of the immune system to avoid detection and thereby cause disease.