Bone marrow is soft tissue located within the hollow interior portion of bones. There are actually two types of marrow: red marrow produces red blood cells, platelets, and most of the white blood cells, while yellow marrow produces a few white blood cell types.
Immune cells differentiate from stem cells in a step-wise fashion. The process begins with hematopoetic stem cells, and from there a single cell differentiates into a range of cell types, including myeloid cells and lymphoid cells. Each of these can further differentiate into more specific cell types – myeloid cells into phagocytes and granulocytes, and lymphoid cells into B cells, T cells, and natural kill cells, among others.
Once they have fully differentiated, lymphocytes exit the blood marrow and travel to other immune organs: T cells to the thymus, and B cells to the spleen. Here, they will undergo further maturation processes. Most other immune cell types leave the bone marrow as fully functional mature cells.
The thymus is a small organ which lies behind the upper part of the sternum, and it is within this organ that immature T cells go through a complex process called thymic selection. Once the process is complete, T cells are fully mature and begin circulating in the bloodstream. At this point, having never been activated by an antigen, they are referred to as naïve cells.
Immature B cells exit the bone marrow and travel to the spleen, where they undergo a maturation process which is entirely different from that which T cells experience, but which is no less important. In both cases, the lymphocytes die if they recognize self antigen, which helps to eliminate the possibility that the system will produce self-reactive cells (when this system fails, the result may be an autoimmune disease).
Lymph Vessels and Nodes
Lymphatic vessels carry a clear fluid called lymph, which bathes the tissues of the body, and also serves as a transport network for immune cells. Lymph nodes are small lumps of tissue which are dotted along the lymphatic system. These nodes are centers of activity where lymphocytes are continually circulating from tissues to lymph nodes and back again, via the bloodstream and lymphatic vessels.
When the body becomes infected, antigen-presenting cells migrate to lymph nodes, where they begin presenting antigens to circulating lymphocytes. If the lymphocytes recognize antigen, they become activated. They stop circulating throughout the body and instead, stay in the lymph node and begin multiplying, thus kicking off the immune response (this is why lymph nodes become swollen and tender as a result of infection).
Other Lymphoid Tissue
There are many other small patches of immune tissue dotted throughout the body, most of which is classified as mucosal-associated lymphoid tissue, known as MALT. This tissue is simply a series of small areas throughout the body where lymphoid tissue is concentrated, but not in a large enough amount to be classified as a lymph node. There are such areas in the gut, bronchus, nose, larynx, and eye, as well as on the skin and in the vascular system.
In addition there are the Peyer’s patches of the small intestine, which facilitate the mucosal immune response, and the tonsils and adenoids, which again, are similar to lymph nodes. The tonsils help protect against pharyngeal and upper respiratory tract infections, while the adenoids provide protection against inhaled pathogens.