Our blood, marrow, and the lymphatic system are closely linked and constantly working together to fuel our bodies.
Blood cells are made inside the spongy tissue in our bones, called bone marrow. The process of blood cell formation is called haematopoiesis. Blood cells start off as stem cells, which are young cells that grow into mature blood cells. Healthy people have enough stem cells to keep on producing new blood cells, continuously.
With the spleen, tonsils, thymus and lymph nodes, the bone marrow forms an internal part of the lymphatic system. The lymphocytes made in the marrow migrate to the lymphatic system and then travel between each node through the lymphatic vessels, which meet at large ducts that empty into a blood vessel. The lymphocytes enter the blood through these ducts.
Blood flows through the marrow to pick up fully formed platelets, red cells and white cells to circulate to the rest of the body.
- Red blood cells (erythrocytes): These transport oxygen around the body.
- White blood cells (leukocytes): These help fight infection and disease. White blood cells include lymphocytes, which make up the cornerstone of the immune system, and myeloid cells, which include granulocytes, neutrophils, monocytes, eosinophils, and basophils.
- Platelets (thrombocytes): These help with blood clotting after injury. Platelets are fragments of the cytoplasm of megakaryocytes, which are another type of bone marrow cell.
How do lymphocytes migrate from the bone marrow to lymph nodes?
There are two routes by which immune cells can enter lymph nodes:
- Leukocytes can arrive from the bloodstream by crossing high endothelial venules. High endothelial venules (HEVs) are blood vessels especially adapted for lymphocyte trafficking which are normally found in secondary lymphoid organs such as lymph nodes (LN) and Peyer’s patches.
- Alternatively, tissue-resident immune cells can enter afferent lymphatic vessels and migrate to draining lymph nodes.
This all sounds very complicated so to summarise:
Bone marrow has stem cells – Stem cells become lymphocytes – Lymphocytes wash into the blood from the bone marrow – Lymphocytes migrate to the thymus or lymph nodes via specially adapted blood capillaries.
You do not need to know more detail than that for the purpose of this course.