Throughout your body are many organs and tissues that play important roles in the immune system. As a group, they’re often referred to as lymphoid tissues or organs. Their main job is to produce and send out the lymphocytes (a type of white blood cell) that target antigens (proteins and other substances) carried by foreign invaders. Some also work to recognize antigens in the bloodstream and then call the lymphocytes into action.
Examples of these organs and tissues include:
- Thymus: a small organ in your upper chest, behind the breastbone, where lymphocytes called T cells grow and mature during childhood. When you reach adulthood, your mature T cells can divide to make new T cells.
- Bone marrow: the soft tissue inside your bones where all blood cells, including lymphocytes, are made. Bone marrow produces T cells and other lymphocytes called B cells.
- Spleen: a fist-sized organ at the upper left of the abdomen, just behind the stomach. The spleen contains white blood cells that respond to any antigens collected from the blood.
- Lymphoid tissue “clumps”: You also have areas of lymphoid tissue throughout the body. Their job is to trap antigens and present them to lymphocytes to trigger an immune response. Lymphoid tissues along the gastrointestinal tract include the tonsils and adenoids, which are located behind the throat and nose, and the appendix, a small organ attached to the large intestine. You also have areas of lymphoid tissue along your respiratory system.
Other important parts of the immune system include lymph vessels and nodes:
- Lymph vessels pick up wastes such as protein, cellular debris, bacteria, and viruses that leak out of the body’s blood vessels. This waste-containing fluid is called lymph, and it travels through the lymph vessels into the lymph nodes. You have lymph vessels throughout your body, in much the same way that you have blood vessels (arteries and veins).
- Lymph nodes are small, round organs that filter out bacteria, waste, and other toxins and also contain infection-fighting white blood cells. The nodes play a key role in recognizing and destroying these substances while also signaling the body to launch an immune response. You have clusters of lymph nodes in your groin, under your arms, and in your neck, as well as more nodes located along other lymphatic pathways in the chest, abdomen, and pelvis.
As the lymph moves out of different areas of the body through the lymph vessels, it slows down to be filtered by the regional lymph nodes. For example, lymph from the hand, arm, and under the arm, as well as the chest and upper back areas, drains to the underarm (also known as axillary) lymph nodes to be filtered.
The lymphoid tissues throughout your body are constantly monitoring your blood and lymph for the presence of foreign substances that could cause harm and require your immune system to take action.