Cancer and the immune system

In a healthy individual, infected or abnormal (mutated) and damaged cells are constantly being identified and destroyed by the immune system. The immune system is a complex system of specialized cells, antibodies, receptors and chemical messengers. It has developed remarkably precise mechanisms that help it to recognize very specific substances called antigens that are displayed (expressed) on the surface of potential invaders like viruses or bacteria or on cells they have infected. Abnormal cells, such as cancer cells can also display specific markers on their surfaces. These are called cancer antigens.

Recognizing ‘foreign’ antigens

The immune response against cancer antigens works in a very similar way to the immune response to cells infected by a virus, for example. If the body detects that a cell is infected, it flags this to the immune system. This happens because every protein produced in a cell is sampled and checked for its origin. When a protein arises from an invader (for example produced by an infecting virus) or is mutated or damaged, it is labelled as ‘foreign’, bound onto a special carrier, called MHC class I molecules (MHC-I) and transported to the cell surface, where it is displayed (expressed) as an antigen.

Once displayed on the cell surface, these antigens can be detected by and reacted against by the immune system. The immune response that is triggered by antigen bound on MHC-I involves special types of white blood cells called T lymphocytes, which are known as cytotoxic T-cells or killer T-cells. These specific types of lymphocytes will bind to the antigen and kill the infected cell that displays it.

The elimination of cancer cells is based on the same principle: a cancer is a mistake in the copying or replication of a cell, which causes abnormal proteins to be produced. These are recognised as ‘foreign’ when they are sampled and are then bound to MHC-I and displayed on the cell surface. Here again, they can be recognized by killer T-cells that will eliminate the cancerous cell before it can reproduce itself.

What goes wrong?

So, in most people, the immune system is the foundation of good health, constantly fighting off every attempt by bacteria or viruses to cause infection and stopping cancer cells from reproducing and growing. There are several reasons why this normal process can go awry, such as a weakened immune system in some people or tumors that are able to evade the immune system. When this happens, cancerous cells may start replicating fast and evolve to a tumor.