What are the organs of the immune system?

Our is made up of both individual cells and proteins as well as entire organs and organ systems. The organs of the immune system include skin and mucous membranes, and the organs of the lymphatic system too.

Organs that function as barriers

Your skin and mucous membranes are the first line of defense against germs entering from outside the body. They act as a physical barrier with support from the following:

  • Antibacterial substances can kill germs right from the start. A certain enzyme found in saliva, the airways and tear fluid destroys the cell walls of .
  • Mucus in the helps trap many of the germs we breathe in so they can be moved out of the airways by hair-like structures called cilia.
  • Stomach acid stops most of the germs that enter the body in the food we eat.
  • Harmless on our skin and many of the mucous membranes in our body also act as part of the .

In addition, the reflexes that cause us to cough and sneeze help to free our airways of germs.

Illustration: The parts of the immune system

The parts of the immune system

Lymphoid organs

The lymphatic system is composed of:

  • Primary lymphoid organs: These organs include the bone marrow and the thymus. They create special cells called lymphocytes.
  • Secondary lymphoid organs: These organs include the lymph nodes, the spleen, the tonsils and certain tissue in various mucous membrane layers in the body (for instance in the bowel). It is in these organs where the cells of the do their actual job of fighting off germs and foreign substances.

Bone marrow

Bone marrow is a sponge-like tissue found inside the bones. That is where most cells are produced and then also multiply. These cells move to other organs and tissues through the blood. At birth, many bones contain red bone marrow, which actively creates cells. Over the course of our life, more and more red bone marrow turns into fatty tissue. In adulthood, only a few of our bones still contain red bone marrow, including the ribs, breastbone and the pelvis.

Thymus

The thymus is located behind the breastbone above the heart. This gland-like organ reaches full maturity only in children, and is then slowly transformed to fatty tissue. Special types of cells called thymus cell lymphocytes (T cells) mature in the thymus. Among other tasks, these cells coordinate the processes of the innate and adaptive immune systems. T cells move through the body and constantly monitor the surfaces of all cells for changes.

Lymph nodes

Lymph nodes are small bean-shaped tissues found along the lymphatic vessels. The lymph nodes act as filters. Various cells trap germs in the lymph nodes and activate the creation of special antibodies in the blood. Swollen or painful lymph nodes are a sign that the is active, for example to fight an .

Spleen

The spleen is located in the left upper abdomen, beneath the diaphragm, and is responsible for different kinds of jobs:

  • It stores various cells. When needed, they move through the blood to other organs. Scavenger cells (phagocytes) in the spleen act as a filter for germs that get into the bloodstream.
  • It breaks down red blood cells (erythrocytes).
  • It stores and breaks down platelets (thrombocytes), which are responsible for the clotting of blood, among other things.

There is always a lot of blood flowing through the spleen tissue. At the same time this tissue is very soft. In the event of severe injury, for example in an accident, the spleen may rupture easily. Surgery is then usually necessary because otherwise there is a danger of bleeding to death. If the spleen needs to be removed completely, other organs can carry out its roles.

Tonsils

The tonsils are also part of the . Because of their location at the throat and palate, they can stop germs entering the body through the mouth or the nose. The tonsils also contain a lot of white blood cells, which are responsible for killing germs. There are different types of tonsils: palatine tonsils, adenoids and the lingual tonsil. All of these tonsillar structures together are sometimes called Waldeyer's ring since they form a ring around the opening to the throat from the mouth and nose.

There is also lymphatic tissue on the side of the throat, which can perform the functions of the palatine tonsils if they are removed.

Mucous membranes

The bowel plays a central role in defending the body against germs: More than half of all the body's cells that produce antibodies are found in the bowel wall, especially in the last part of the and in the appendix. These cells detect foreign substances, and then mark and destroy them. They also save information about the substances in order to be able to react more quickly the next time. The large bowel also contains harmless called gastrointestinal or gut flora. Healthy gut flora make it difficult for germs to spread and enter the body.

Mucous membranes support the in other parts of the body, too, such as the respiratory and urinary tracts, and the lining of the vagina. The cells are directly beneath the mucous membranes, where they prevent bacteria and viruses from attaching.

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.

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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.

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Updated on July 30, 2020
Next planned update: 2023

Authors/Publishers:

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

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