Bone marrow is a sponge-like tissue found inside the bones. That is where most immune system 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 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.
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 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 are small bean-shaped tissues found along the lymphatic vessels. The lymph nodes act as filters. Various immune system 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 immune system is active, for example to fight an infection.
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 small bowel 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 bacteria called gastrointestinal or gut flora. Healthy gut flora make it difficult for germs to spread 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, where they prevent bacteria and viruses from attaching.
The tonsils are also part of the immune system. 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.
The spleen is located in the left upper abdomen, beneath the diaphragm, and is responsible for different kinds of jobs:
It stores various immune system 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 immune system organs can carry out its roles.
Primary lymphoid organs: These organs include the bone marrow and the thymus. They create special immune system 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)
Plasmin is a serine protease also referred to as fibrin digesting enzyme, it is responsible for the dissolving or degradation of blood clot. The process by which plasmin degrades blood clot is called fibrinolysis. Note that plasmin (active) is formed from an inactive enzyme called plasminogen.
