LYMPH NODES

• Secondary organs of the immune system. These small bean-shaped structures are found where blood and lymph vessels converge, such as the axillary and groin areas.

Overview of Structures:

Capsule

• Gives rise to trabeculae, which divide the node into sections and provide passage for blood vessels.
  Afferent lymphatic vessels
• Pierce the capsule to deliver lymphatic fluid to the node; valves promote unidirectional flow.
  Hilum
• Area of indentation of the node; the efferent lymphatic vessel drains lymph from the node at the hilum.
• Blood vessels also enter and exit at the hilum.
  Medulla
• Open to the hilum
  Cortex
• Lies just beneath the capsule.

Stroma

Reticular fibers and cells

Lymphatic tissues of Lymph Node

• Lymphatic tissues are responsible for filtering and processing antigens present in the lymphatic fluid as it travels from the afferent vessels to the efferent vessel.
• Lymphatic tissues are densely packed in the cortex
• Reside more loosely in the medulla as the medullary cords.

Cortex

• Outer cortex
  • Aggregations of B cells form primary follicles.
  • When the B cells proliferate, they produce secondary follicles, which comprise a lighter-staining germinal center and the darker-staining mantle.
  • The light and dark areas reflect lymphocyte size: the germinal centers comprise active medium-sized loosely organized lymphocytes, and the mantle comprises smaller lymphocytes with condensed chromatin.
  • In addition to lymphocytes, macrophages and follicular dendritic cells (FDC) reside within the follicles;
    Dendritic cells are the primary antigen-presenting cells (APC); they present microbial antigens on their surfaces to trigger T cell activation.
• Inner cortex
  • T cells and dendritic cells

Sinuses and Medullary cords

• Macrophages
• Additional lymphocytes

Route of lymph fluid through Sinuses

• Sinuses are lined by endothelial cells

1. Lymph fluid passes through the afferent lymphatic vessel into the sub-capsular, aka, marginal sinus
2. Travels through the cortical, aka, trabecular or para-trabecular sinus, to the medullary sinus
3. Exits medullary sinus through efferent lymphatic vessel.
4. From there, it is transported in the lymphatic vessels to other lymph nodes and eventually returned to the blood.