Medical Health Encyclopedia

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 either are filtered out or are added to the contents of the node.

Lymph Nodes

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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.