Organs of the Immune System: The Bone Marrow

Among the organs that contribute to our immune response, there are two categories: primary lymphoid organs and secondary lymphoid organs.

The bone marrow is one of two primary lymphoid organs, along with the thymus. Bone marrow is a sponge-like tissue found at the center of the body's large, flat bones.

The Job of the Bone Marrow

Bone marrow is where B-cells are selected, developed and matured in a process called hematopoiesis. Two types of stem cells are made in the bone marrow: stromal cells, which make fat, cartilage and bone, and hematopoietic stem cells, which make blood cells. Furthermore, two types of marrow are found in bone marrow: red marrow, where blood cells and platelets come from, and yellow marrow, which has a high amount of fat cells and makes some white blood cells.

Like we saw in the thymus, at birth there is almost exclusively red marrow, but as we age, more of the red marrow converts into the yellow marrow, with the high fat content.

Bone Marrow in Lymphoma

Because the bone marrow is where hemopoietic stem cells are made, it plays a very important role in lymphoma. When you consider how important the job of the bone marrow is — creating the cells that will become the blood cells that carry oxygen through the body and remove carbon dioxide from tissue, defending the body against infection and disease, and helping the process of clotting so that we don't bleed to death with a simple cut — you can begin to see how devastating any bone marrow disease can be. After all, blood cells have a short life span; they are always being replaced, and this production process is of crucial importance in maintaining not just good health, but life itself.

In diseases of the bone marrow, stem cells might produce too many, too few, or abnormal and non-functional blood cells. If the white blood cells being made in the bone marrow are abnormal, that is characteristic of leukemia. If the bone marrow isn't making red blood cells, it is called aplastic anemia.

Lymphomas, however, don't begin in the bone marrow, but they can spread to the bone marrow and disrupt the production of blood cells and platelets. Almost all subtypes of lymphoma can spread to the bone marrow, which is why a bone marrow biopsy is often part of the diagnostic and prognostic process in lymphoma.

When lymphoma proves especially difficult to treat, doctors may perform a bone marrow transplant, either from the patient or from a donor. The hope in such a transplant is to remake the bone marrow so that it can build a new immune system following high-dose chemotherapy and radiation that is designed to kill all the cancer cells in the body. Since this process kills the bone marrow too, a transplant is necessary.