Bone marrow

Bone marrow is a sponge-like tissue found inside the bones. That’s where most immune system cells are made and then also multiply. These cells move to other organs and tissues through the bloodstream. At birth, many bones contain red bone marrow, which actively creates immune system 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 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 immune system 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 clumps of tissue found along the lymphatic vessels. The lymph nodes act as filters. Various immune system cells trap germs in the lymph nodes and trigger the production of special antibodies in the blood. Swollen or painful lymph nodes are a sign that the immune system is active – for example, because it’s fighting an infection.

Spleen

The spleen is found in the upper left abdomen, beneath the diaphragm, and is responsible for many different jobs:

• It stores various immune system cells. When needed, these cells travel to other organs in the bloodstream. 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.

Spleen tissue always has a lot of blood flowing through it. It is also very soft, so it can tear easily – for example, when injured in an accident. Surgery is then usually needed because otherwise there’s a risk of bleeding to death. If the spleen needs to be removed completely, other immune system organs can carry out its job instead.

Tonsils

The tonsils are also part of the immune system. Because of their location in the throat and roof of the mouth, 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. Together, they form a ring of tonsil tissue (known as Waldeyer’s ring) around the opening of the throat from the mouth and nose.

There is also lymphoid tissue on the side of the throat. This tissue can take over the job of the palatine tonsils if they are removed.

Mucous membranes

The intestine (bowel) plays a central role in defending the body against germs: More than half of all the body's cells that make antibodies are found in the wall of the intestine, especially in the last part of the small intestine and in the appendix. These cells detect foreign substances, and then mark and destroy them. They also store information about the substances so they can react more quickly the next time. The large intestine also contains harmless bacteria called gut flora. Healthy gut flora make it difficult for germs to settle and enter the body.

Mucous membranes support the immune system in other parts of the body, too – such as the respiratory and urinary tracts, and the lining of the vagina. The immune system cells are directly beneath the mucous membranes. They prevent bacteria and viruses from settling and growing there.