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What is an inflammation?

Created: ; Last Update: February 22, 2018; Next update: 2021.

When a wound swells up, turns red and hurts, it may be a sign of inflammation. Inflammation is – very generally speaking – the body’s immune system’s response to stimulus. This can be bacteria colonizing a wound or a splinter piercing your finger, for example. Inflammation happens when the immune system fights against something that may turn out to be harmful.

Causes of an inflammation

Inflammation may have many different causes. These are the most common:

  • Pathogens (germs) like bacteria, viruses or fungi
  • External injuries like scrapes or foreign objects (for example a thorn in your finger)
  • Effects of chemicals or radiation

Diseases or conditions that cause inflammation often have a name ending in “-itis.” For example:

  • Cystitis, an inflammation of the bladder
  • Bronchitis, an inflammation of the bronchi 
  • Otitis media, a middle ear infection 
  • Dermatitis, a disease where the skin is inflamed

Signs of an inflammation

There are five signs that may indicate an acute inflammation:

  • Redness
  • Heat
  • Swelling
  • Pain
  • Loss of function

There is a loss of function, for example, when the inflamed limb can no longer be moved properly or when the sense of smell is worse during a cold, or when it is more difficult to breathe when you have bronchitis.

This means that an inflammation does not start when a wound has been infected by bacteria, festers, or heals poorly, but already as the body is trying to fight against the harmful stimulus or a viral infection.

Not all five signs occur in every inflammation. Some inflammations occur “silently” and do not cause any symptoms.

The body’s general response

If the inflammation is severe, it may cause general reactions in the body. This may include the following signs and symptoms:

  • General symptoms of feeling sick, exhaustion and fever: These symptoms are a sign that the immune defense is very active and needs a lot of energy, which may be lacking for other activities. If the rate of metabolism is higher due to a fever, more defense substances and cells can be produced.
  • Changes in the blood such as an increased number of defense cells.

A very rare but dangerous complication of an inflammation is called sepsis. Sepsis may occur if bacteria multiply quickly in a certain part of the body and then suddenly enter the bloodstream in large quantities. This can happen if the body does not succeed in fighting the inflammation locally, the pathogens are very aggressive, or the immune system is severely weakened.

Chills, feeling very ill, and very high fever can also be signs of blood poisoning. If blood poisoning is suspected, medical assistance is urgently needed.

What happens when you have an inflammation

Many different immune cells can take part in an inflammation. They release different substances, the inflammatory mediators. These include the tissue hormones bradykinin and histamine. They cause the narrow blood vessels in the tissue to expand, allowing more blood to reach the injured tissue. For this reason the inflamed area turns red and becomes hot.

More defense cells are also brought along with the blood to the injured tissue, to help with the healing process. Both hormones can also irritate nerves and cause pain signals to be sent to the brain. If the inflammation hurts, you usually favor the affected part of the body.

The inflammatory mediators have yet another function: they increase the permeability of the narrow vessels, so that more defense cells can enter the affected tissue. The defense cells also carry more fluid into the inflamed tissue, which is why it often swells up. After this fluid is transported out of the tissue once again a while later and the swelling disappears again.

The mucous membranes also release more fluid during inflammation. This happens for example when you have a stuffy nose and the nasal mucous membranes are inflamed. Then the nasal secretions can help to quickly flush the viruses out of the body.

Inflammations can also cause chronic diseases

An inflammation is not always a helpful response of the body. In certain diseases the immune system fights against its own cells by mistake, causing harmful inflammatory responses. These include, for example:

  • Rheumatoid arthritis, where many joints throughout the entire body are permanently inflamed
  • Psoriasis, a chronic skin disease
  • Inflammations of the bowel like Crohn’s disease or ulcerative colitis

These diseases are called chronic inflammatory diseases, and can last for years or even a lifetime in varying degrees of severity and activity.


  • Andreae S. Lexikon der Krankheiten und Untersuchungen. Stuttgart: Thieme; 2008.
  • Kaspers DL, Hauser SL, Jameson JL, Fauci AS, Longo DL, Loscalzo J. Harrison’s Principles of internal medicine. New York: McGraw-Hill Companies. 19th ed; 2015.
  • Pschyrembel W. 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. 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: NBK279298


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