NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health. [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-.

Cover of [Internet].

Show details

How does the immune system work?

Created: ; Last Update: September 21, 2016; Next update: 2019.

The immune system (from the Latin word immunis, meaning: “free” or “untouched”) protects the body like a guardian from harmful influences from the environment and is essential for survival. It is made up of different organs, cells and proteins and aside from the nervous system, it is the most complex system that the human body has.

As long as our body’s system of defense is running smoothly, we do not notice the immune system. And yet, different groups of cells work together and form alliances against just about any pathogen (germ). But illness can occur if the performance of the immune system is compromised, if the pathogen is especially aggressive, or sometimes also if the body is confronted with a pathogen it has not come into contact before.

The tasks of the immune system

Without an immune system, a human being would be just as exposed to the harmful influences of pathogens or other substances from the outside environment as to changes harmful to health happening inside of the body. The main tasks of the body’s immune system are:

  • Neutralizing pathogens like bacteria, viruses, parasites or fungi that have entered the body, and removing them from the body
  • Recognizing and neutralizing harmful substances from the environment
  • Fighting against the body’s own cells that have changed due to an illness, for example cancerous cells

Differentiation between self and non-self substances

For protection to be effective it is important, however, that the immune system can differentiate between “self” and “non-self” cells, organisms and substances. Usually, the body should not work against its own healthy cells.

The immune system can be activated by many “non-self” substances. These are called antigens. The proteins on the surfaces of bacteria, fungi and viruses, for example, are all antigens. When the antigens bind to, for example, special receptors on the defense cells, a series of cell processes is started. Then the immune system can recall stored “memories” in order to more quickly be ready to defend against known pathogens.

The body’s own cells have surface proteins, too. But the immune system does not work against them, because it has already learned at an earlier stage to identify specifically these cell proteins as “self.” If the immune system identifies the cells of its own body as “non-self,” it is also called an autoimmune reaction.

Innate immune system

There are two main parts of the immune system: the innate and the adaptive immune system.

The evolutionary older innate immune system provides a general defense against pathogens, so it is also called the nonspecific immune system. It works mostly at the level of immune cells like “scavenger cells” or “killer cells.” These cells mostly fight against bacterial infections.

Adaptive immune system

In the adaptive immune system, particular agents like the so-called antibodies target very specific pathogens that the body has already had contact with. That is why this is also called a learned defense or a specific immune response. By constantly adapting and learning the body can also fight against bacteria or viruses that change over time.

Yet these two immune systems do not work independently of each other. They complement each other in any reaction to a pathogen or harmful substance, and are closely connected with each other.


  • Menche N. (ed.) Biologie Anatomie Physiologie. Munich: Urban & Fischer/ Elsevier; 2012.
  • Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.
  • Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer; 2011.
  • 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. 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.

© IQWiG (Institute for Quality and Efficiency in Health Care)
Bookshelf ID: NBK279364


Informed Health Links

Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...