How does the immune system work?

Without an immune system, we would have no way to fight harmful things that enter our body from the outside or harmful changes that occur inside our body. The main jobs of the body’s immune system include

• fighting disease-causing germs (pathogens) like bacteria, viruses, parasites or fungi, and to remove them from the body,
• recognizing and neutralizing harmful substances from the environment, and
• fighting disease-causing changes in the body, such as cancer cells.

The immune system can be activated by a lot of different things that the body doesn’t recognize as its own. These are called antigens. Examples of antigens include the proteins on the surfaces of bacteria, fungi and viruses. When these antigens attach to special receptors on the immune cells ( immune system cells), a whole series of processes are triggered in the body. Once the body has come into contact with a disease-causing germ for the first time, it usually stores information about the germ and how to fight it. Then, if it comes into contact with the germ again, it recognizes the germ straight away and can start fighting it faster.

The body’s own cells have proteins on their surface, too. But those proteins don’t usually trigger the immune system to fight the cells. Sometimes the immune system mistakenly thinks that the body's own cells are foreign cells. It then attacks healthy, harmless cells in the body. This is known as an autoimmune response.

There are two subsystems within the immune system, known as the innate (non-specific) immune system and the adaptive (specific) immune system. Both of these subsystems are closely linked and work together whenever a germ or harmful substance triggers an immune response.

The innate immune system provides a general defense against harmful germs and substances, so it’s also called the non-specific immune system. It mostly fights using immune cells such as natural killer cells and phagocytes (“eating cells”). The main job of the innate immune system is to fight harmful substances and germs that enter the body, for instance through the skin or digestive system.

The adaptive (specific) immune system makes antibodies and uses them to specifically fight certain germs that the body has previously come into contact with. This is also known as an “acquired” (learned) or specific immune response.

Because the adaptive immune system is constantly learning and adapting, the body can also fight bacteria or viruses that change over time.