The lymphatic system and the first lymphoid organs
The human immune system is made of organs, tissues, cells and chemicals that work together. Its main part is the lymphatic system: lymph, lymph vessels and the lymphoid organs. The other parts are the white blood cells, chemicals that help them, and the antibodies made by some of them. Lymphocytes, an important kind of white blood cell, are produced, mature and differentiate in the primary lymphoid organs, which are the bone marrow and the thymus gland. The secondary lymphoid organs are the spleen, the tonsils, Peyer’s patches, the appendix and the lymph nodes, and they hold large numbers of lymphocytes. The bone marrow is tissue inside the flat bones, such as the breastbone, skull, vertebrae, ribs, shoulder and pelvis, and in the heads of the long bones, such as the femur, tibia and humerus. It makes all kinds of white blood cells and matures all of them except the T lymphocytes. The thymus gland lies on the windpipe above the heart and behind the breastbone. Its hormone, thymosin, makes lymphoid stem cells mature into T cells inside the gland and become different types. Because thymosin is a hormone, the thymus belongs to the immune system and to the glands you met in Chapter 2. The spleen is about the size of the palm, dark red, and lies in the upper left of the abdomen. Its macrophages pick up microbes, foreign bodies and old red blood cells and break them into parts that the body can dispose of, and its lymphocytes release antibodies. The tonsils are two lymphoid glands, one on each side at the back of the mouth. They catch microbes and foreign bodies that enter with food or air. Peyer’s patches, the appendix and the lymph nodes come next week.
| Organ | Kind | Where | Job |
|---|---|---|---|
| Bone marrow | primary | inside flat bones and the heads of long bones | makes white blood cells; matures all but T cells |
| Thymus gland | primary | on the windpipe, above the heart, behind the breastbone | thymosin matures T cells |
| Spleen | secondary | upper left of the abdomen | macrophages clear microbes and old red cells; lymphocytes release antibodies |
| Tonsils | secondary | back of the mouth, one on each side | catch microbes entering with food or air |
| Job | Organ |
|---|---|
| matures T cells | thymus gland |
| clears old red blood cells | spleen |
| makes all white blood cells | bone marrow |
| catches microbes entering with food or air | tonsils |
Body map, with a grown-up. Draw a simple outline of a body. Mark roughly where the bone marrow (in a flat bone and in the head of a long bone), the thymus gland, the spleen and the tonsils lie, using only the places named in the lesson. Colour the primary organs red and the secondary organs blue, and write one job beside each.