How does the immune system work?

The has a vital role: It protects your body from harmful substances, germs and cell changes that could make you ill. It is made up of various organs, cells and proteins.

As long as your is running smoothly, you don’t notice that it’s there. But if it stops working properly – because it’s weak or can't fight particularly aggressive germs – you get ill. Germs that your body has never encountered before are also likely to make you ill. Some germs will only make you ill the first time you come into contact with them. These include childhood diseases like chickenpox.

The tasks of the immune system

Without an , 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 tasks of the body’s are

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

How is the immune system activated?

The 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 , fungi and viruses. When these antigens attach to special receptors on the immune cells ( 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 to fight the cells. Sometimes the 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.

Innate and adaptive immune system

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

The adaptive (specific) 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 is constantly learning and adapting, the body can also fight or viruses that change over time.

Brandes R, Lang F, Schmidt R (Ed). Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.

Menche N (Ed). Biologie Anatomie Physiologie. München: Urban und Fischer; 2016.

Pschyrembel. Klinisches Wörterbuch. Berlin: De Gruyter; 2017.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Updated on April 23, 2020
Next planned update: 2023

Authors/Publishers:

Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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