Lymph nodes are small nodules of lymphoid tissue found in the course of smaller lymphatics. The lymph passes through one or more lymph nodes before reaching the larger lymph trunks. The nodes are oval or reniform in shape, 1-25 mm long, and light brown, black (pulmonary), or creamy white (intestinal) in color. Usually they occur in groups (cervical, axillary, inguinal, mesenteric, mediastinal, etc.), but at times there may be a solitary lymph node.
Each lymph node has a slight depression on one side, called hilum. The artery enters the node, and the vein with efferent lymphatic comes out of it, at the hilum. The afferent lymphatics enter the node at different parts of its periphery.
Structure of a lymph node:
Structurally, a lymph node is made up of the following parts.
Fibrous and reticular framework:
The lymph node is covered by a capsule. From the deep surface of the capsule a number of trabeculae extend radially into the interior of the node, where they are continuous with the fine reticulum which forms the supporting framework for the lymphoid tissue.
The sub-capsular sinus lies beneath the capsule and surrounds the node except at the hilum. Many afferent lymphatics of the node open into the sub-capsular sinus. Lymph filters through reticulin fibers and leaves the node by one efferent lymphatic vessel.
It is the outer part of the lymph node situated beneath the sub-capsular sinus, being absent at the hilum.
It is made up of lymphatic follicles and is traversed by fibrous trabcculae. The cortex is far more densely cellular than the medulla. It is divided into:
- Zone 1. containing loosely packed small lymphocytes, macrophages and occasional plasma cells in the periphery of the follicle and extending into the medullary cords.
- Zone 2, containing more densely packed small lymphocytes and macrophages, deep to zone I and limited to cortical and paracortical (inner cortex) areas.
- Zone 3, including the germinal centre which contains large lymphocytes and macrophages.
The maturing lymphocytes pass from zone 3 to zone 2 to zone 1 and to the lymph sinus.
According to the distribution of B- and T-Lymphocytes, the cortex is divided into:
- An outer part which contains immature B-lymphocytes.
- An inner part, between the germinal centre and the medulla, which contains T-lymphocytes. This part is known as paracortex or thymus dependent zone.
The mature B-lymphocytes (plasma cells) are found in the medulla.
It is the central part of the lymph node, containing loosely packed lymphocytes (forming irregular branching medullary cords), the plasma cells, and macrophages.
The artery enters at the hilum and divides into straight branches which run in the trabeculae. In the cortex the arteries further divide to form arcades of arterioles and capillaries with many anastomosing loops.
The capillaries give rise to venules and veins, which run back to the hilum. The capillaries are more profuse around the follicles, and the postcapillary venules are more abundant in the paracortical zones for lymphatic migration.