The Anatomy of the Immune System Explained

Lymphocytes, Lymph Nodes and the Spleen Make up the Immune System

Antibodies attacking a virus, artwork
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The organs of the immune system are stationed throughout the body. They are generally referred to as lymphoid organs because they are concerned with the growth, development, and deployment of lymphocytes, the white cells that are the key operatives of the immune system. Lymphoid organs include the bone marrow and the thymus, as well as lymph nodes, spleen, tonsils and adenoids, the appendix, and clumps of lymphoid tissue in the small intestine known as Peyer's patches.

The blood and lymphatic vessels that carry lymphocytes to and from the other structures can also be considered lymphoid organs.

Cells destined to become immune cells, like all other blood cells, are produced in the bone marrow, the soft tissue in the hollow shafts of long bones. The descendants of some so-called stem cells become lymphocytes, while others develop into a second major group of immune cells typified by the large, cell-and particle-devouring white cells known as phagocytes.

B-Cells and T-Cells

The two major classes of lymphocytes are B cells and T cells. Whereas B cells complete their maturation in the bone marrow, T cells migrate to the thymus, a multilobed organ that lies high behind the breastbone. In the thymus, the T cells multiply and mature into cells capable of producing immune response. In other words, T cells become immunocompetent in the thymus. In a process referred to as T cell "education," T cells in the thymus learn to distinguish self cells from non-self cells; T cells that would react against self antigens are eliminated.

On exiting the bone marrow and thymus, some lymphocytes congregate in immune organs or lymph nodes. Others cells--both B and T cells--travel widely and continuously throughout the body. These cells travel using the blood circulation as well as a body-wide network of lymphatic vessels similar to blood vessels.

The Lymph Nodes, Lymph and Spleen

Laced along the lymphatic routes--with clusters in the neck, armpits, abdomen, and groin--are small, bean-shaped lymph nodes. Each lymph node contains specialized compartments that house platoons of B lymphocytes, T lymphocytes, and other cells capable of enmeshing antigen and present the antigen to T cells. Thus, the lymph node brings together the several components needed to spark an immune response.

The spleen, too, provides a meeting ground for immune defenses. A fist-sized organ at the upper left of the abdomen, the spleen contains two main types of tissue: the red pulp that disposes of worn-out blood cells and the white pulp that contains lymphoid tissue. Like the lymph nodes, the spleen's lymphoid tissue is subdivided into compartments that specialize into different kinds of immune cells. Microorganisms carried by the blood into the red pulp become trapped by the immune cells known as macrophages. (Although people can live without a spleen, such as people whose spleens have been damaged by trauma or by disease such as sickle cell anemia, these people are highly susceptible to infection; surgical removal of the spleen is especially dangerous for young children and the immunosuppressed.)

Nonencapsulated clusters of lymphoid tissue are found in many parts of the body. These clusters are common around the mucous membranes lining the respiratory and digestive tracts, or areas that serve as gateways to the body. These nonencapsulated clusters include the tonsils and adenoids, the appendix, and Peyer's patches.

The lymphatic vessels carry lymph, a clear fluid that bathes the body's tissues. Lymph, along with the many cells and particles it carries--notably lymphocytes, macrophages, and foreign antigens--drains out of tissues and seeps across the thin walls of tiny lymphatic vessels. The vessels transport the mix to lymph nodes, where antigens can be filtered out and presented to immune cells.

Additional lymphocytes reach the lymph nodes (and other immune tissues) through the bloodstream. Each node is supplied by an artery and a vein; lymphocytes enter the node by traversing the walls of the very small specialized veins 

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