Lymphatic System: Functions and Components

Lymphatic System

• Consists of three parts: lymphatic vessels, lymph, and lymph nodes.
• Returns interstitial fluid and leaked plasma proteins back to the blood.
• Provides the structural basis of the immune system with lymphoid organs and tissues.

Lymph Transport

• Lymph is propelled by pulsations of nearby arteries and contractions of smooth muscle in the walls of lymphatics.

Lymphoid Cells

• Lymphocytes are the main warriors of the immune system.
• Two main varieties: T cells (T lymphocytes) and B cells (B lymphocytes).
• T cells manage the immune response and attack and destroy foreign cells.
• B cells produce plasma cells, which secrete antibodies.

Other Lymphoid Cells

• Macrophages phagocytize foreign substances and help activate T cells.
• Dendritic cells capture antigens and deliver them to lymph nodes.
• Reticular cells produce stroma that supports other cells in lymphoid organs.

Lymphoid Tissue

• Houses and provides a proliferation site for lymphocytes.
• Furnishes a surveillance vantage point.
• Two main types: diffuse lymphatic tissue and lymphatic follicles.
• Diffuse lymphatic tissue comprises scattered reticular tissue elements in every body organ.
• Lymphatic follicles are solid, spherical bodies of tightly packed reticular elements and cells.

Lymph Nodes

• Principal lymphoid organs of the body.
• Embedded in connective tissue, in clusters along lymphatic vessels.
• Near the body surface in inguinal, axillary, and cervical regions of the body.

Thymus

• Differs from other lymphoid organs in important ways.
• Functions strictly in T lymphocyte maturation.
• Does not directly fight antigens.
• The stroma of the thymus consists of star-shaped epithelial cells (not reticular fibers).

Tonsils

• Simplest lymphoid organs.
• Form a ring of lymphatic tissue around the pharynx.
• Contain follicles with germinal centers.
• Are not fully encapsulated.
• Epithelial tissue overlying tonsil masses invaginates, forming tonsillar crypts.

Aggregates of Lymphoid Follicles

• Peyer’s patches: clusters of lymphoid follicles in the wall of the distal portion of the small intestine.
• Similar structures are also found in the appendix.
• Destroy bacteria, preventing them from breaching the intestinal wall.
• Generate “memory” lymphocytes.

MALT

• Mucosa-associated lymphatic tissue, including Peyer’s patches, tonsils, and the appendix (digestive tract).
• Lymphoid nodules in the walls of the bronchi (respiratory tract).
• Protects the digestive and respiratory systems from foreign matter.

Developmental Aspects

• Beginnings of the lymphatic vessels and main clusters of lymph nodes are apparent by the 5th week of embryonic development.
• These arise from the budding of lymph sacs from developing veins.
• Lymphatic organs (except the thymus) arise from mesoderm.
• The thymus (endodermal origin) forms as an outgrowth of the pharynx.
• Except for the spleen and tonsils, lymphoid organs are poorly developed at birth.