The Lymphatic System

Lymphomas, such as non-Hodgkin's lymphomas and Hodgkin's disease, represent tumors of the lymphatic system. This system is a network of organs, ducts, and nodes that interacts with the blood's circulatory system to transport a watery clear fluid called lymph throughout the body. The lymphatic system contains lymphocytes, which are important cells involved in defending the body against infectious organisms. This system also restores 60% of the fluid that leaks out from blood capillaries back into circulation, and its ducts provide transportation for fats, proteins, and other substances collected from the body's tissues.

Lymphocytes

The lymphatic system is involved in the production and transportation of lymphocytes, white blood cells that are a primary component of the immune system. Among other vital functions, certain lymphocytes are responsible for producing antibodies , factors that can target and attack specific foreign proteins (antigens). To understand the lymphatic system, it is helpful to track part of the life cycle of these lymphocytes:

• Lymphocytes develop in the bone marrow or thymus gland and are therefore categorized as either B cells (bone marrow-derived cells) or T cells (thymus gland-derived cells).
• B cells complete their structural growth and definition (known as differentiation) and mature in the bone marrow.
• T cells also start out in the bone marrow but differentiate and mature in the thymus gland , located beneath the breastbone ( sternum ). This small gland is active mostly in the fetal stage through the first 10 years of life, after which it atrophies (shrinks).
• B-cell and T-cell lymphocytes leave these organs through the bloodstream, which eventually branches out into the tiny blood vessels called capillaries.
• Some lymphocytes, along with fluid, proteins, and other substances, migrate out of the capillaries into the surrounding tissues. A proportion of these lymphocytes and other substances then enter the lymphatic vessels .
• Lymphatic vessels begin as tiny, blind-ended tubes and lead to larger lymphatic ducts and branches until they drain into two ducts in the neck, where the fluid re-enters the bloodstream.
• Along the way, the fluid passes through lymph nodes , which are oval structures composed of lymph vessels, connective tissue, and white blood cells. Here, the lymphocytes are either filtered out or added to the contents of the node.

Lymph Nodes

The lymph node provides an environment where lymphocytes can receive their initial exposure to foreign substances (antigens) such as bacteria or other microorganisms, which then activates the lymphocytes to perform their immune functions. The size of a lymph node varies generally from that of a pinhead to a bean. Most nodes are in clusters located throughout the system. Important node clusters are found in the neck, lower arm, armpit, and groin.

Other Structures in the Lymphatic System

The tonsils and adenoids are secondary organs composed of masses of lymph tissue that also play a role in the lymphatic system. The spleen is another important organ that processes lymphocytes from incoming blood.

B-cell Lymphomas Classified by Grade

Indolent (Slow-Growing) Lymphomas (also referred to as Low-Grade Lymphomas)

Follicular lymphomas (FLs) . Follicular small cleaved cell lymphoma (grade I) and follicular mixed small and large cell lymphoma (grade II). FLs account for 70% of indolent tumors and 20% of all NHLs in industrialized countries. It is very rare in developing countries and in Asia.

Lymphoplasmacytoid/Waldenstrom's macroglobulinemia . Often found in bone marrow, lymph nodes, and spleen. Can cause blood to become viscous and "sticky."

Marginal zone lymphomas (MZL) . MZLs often occur as a result of a pre-existing disorder such as hepatitis C, bacterial infection in the stomach ( H. pylori ), or an autoimmune disorder (Sjögren syndrome in the salivary glands or Hashimoto's thyroiditis in the thyroid gland). They may be classified as:

• Monocytoid B-cell lymphoma, which involves only lymph nodes
• Splenic marginal zone lymphoma, which affects the spleen, blood, and bone marrow
• Mucosa-associated lymphoid tissue (MALT) lymphoma, which usually involves the gastrointestinal tract, thyroid, lung, breast, or skin

There is some controversy over whether MALT is a variation of MZL or a completely separate type of lymphoma that is more suitably classified as a separate low-grade lymphoma. At this time, it is classified as an MZL.

Aggressive Lymphomas (also referred to as Intermediate- and High-Grade Lymphomas)

Diffuse large-cell lymphomas (DL). DLs are the most common NHLs, accounting for about 40% of all cases. Subtypes include the following:

• Primary mediastinal large B-cell lymphoma
• Follicular large cell lymphoma
• Anaplastic large cell lymphoma
• T-cell lymphomas (not covered in this report)

In about 40% of cases, these DL lymphomas appear in areas outside lymph nodes, including digestive tract, skin, bone, thyroid, and testes.

Burkitt's lymphoma/diffuse, small noncleaved cell lymphoma . This is the most common childhood NHL. In African children, it often involves facial bones and is associated with Epstein-Barr infection.

Mantle cell lymphoma . Mantle cell lymphomas are found in lymph nodes, the spleen, bone marrow, blood, and sometimes the gastrointestinal system (lymphomatous polyposis). This lymphoma is similar to indolent lymphomas at the time of diagnosis, but it is more aggressive.

Lymphoblastic lymphoma . This lymphoma often occurs in young people. It is associated with a large mediastinal mass (occurring in chest cavity between the lungs) and carries a high risk for spreading to bone marrow and central nervous system.