Blood Cell Lines and the Lymph System

Blood Cell Lines

In adults, blood cells are produced by the bone marrow, the spongy material filling the body's bones. The bone marrow produces two blood cell groups, myeloid and lymphoid.

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Myeloid Cell Line. The myeloid cell line includes the following:

• Immature cells called erythrocytes that later develop into red blood cells
• Blood clotting cells (platelets)
• Some white blood cells, including macrophages (which act as scavengers for foreign particles), eosinophils (which trigger allergies and also defend against parasites), and neutrophils (the main defenders against bacterial infections)

Lymphoid Cell Line. The lymphoid cell line includes the lymphocytes, which are the body's primary infection fighters. Among other vital functions, certain lymphocytes are responsible for producing antibodies, factors that can target and attack specific foreign substances (antigens).

Lymphocytes develop in the thymus gland or bone marrow and are therefore categorized as either B cells (bone marrow-derived cells) or T cells (thymus gland-derived cells).

Lymphocytes and the Lymph System

To understand how acute lymphocytic leukemia (ALL) arises requires knowledge of lymphocytic development and function:

• B cells develop and mature in their final form (known as differentiation) 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. This small gland is active mostly in the fetal stage through the first ten 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.
• Once they leave the capillaries, some lymphocytes migrate into the surrounding tissues. A proportion of these lymphocytes (along with fluid, proteins, and other substances) then enters the lymphatic vessels.
• Lymphatic vessels begin as tiny, blind-ended tubes and lead to larger lymphatic ducts and branches. 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 are added to the contents of the node.