The Lymph Nodes

Lymph nodes are kidney shaped structures which act to filter foreign particles from the blood, and play an important role in the immune response to infection. On average, an adult has around 400 to 450 different lymph nodes spread throughout the body – with the majority located within the abdomen.

Each node contains T lymphocytes, B lymphocytes, and other immune cells. They are exposed to the fluid as it passes through the node, and can mount an immune response if they detect the presence of a pathogen. This immune response often recruits more inflammatory cells into the node – which is why lymph nodes are palpable during infection.

Lymph fluid enters the node through afferent lymphatic channels and leaves the node via efferent channels. Macrophages located within the sinuses of the lymph node act to filter foreign particles out of the fluid as it travels through.

Lymph nodes are kidney or oval shaped and range in size from 0.1 to 2.5 cm long. Each lymph node is surrounded by a fibrous capsule, which extends inside a lymph node to form trabeculae. The substance of a lymph node is divided into the outer cortex and the inner medulla. These are rich with cells. The hilum is an indent on the concave surface of the lymph node where lymphatic vessels leave and blood vessels enter and leave,

Lymph enters the convex side of a lymph node through multiple afferent lymphatic vessels and from here flows into a series of sinuses. After entering the lymph node from afferent lymphatic vessels, lymph flows into a space underneath the capsule called the subcapsular sinus, then into cortical sinuses, After passing through the cortex, lymph then collects in medullary sinuses. All of these sinuses drain into the efferent lymph vessels to exit the node at the hilum on the concave side.

Location

Lymph nodes are present throughout the body, are more concentrated near and within the trunk, and are divided into groups. There are about 450 lymph nodes in the adult. Some lymph nodes can be felt when enlarged (and occasionally when not), such as the axillary lymph nodes under the arm, the cervical lymph nodes of the head and neck and the inguinal lymph nodes near the groin crease. Most lymph nodes lie within the trunk adjacent to other major structures in the body – such as the paraaortic lymph nodes and the tracheobronchial lymph nodes.

There are no lymph nodes in the central nervous system, which is separated from the body by the blood-brain barrier. Lymph from the meningeal lymphatic vessels in the CNS drains to the deep cervical lymph nodes.

Cells

Subdivisions

A lymph node is divided into compartments called nodules (or lobules), each consisting of a region of cortex with combined follicle B cells, a paracortex of T cells, and a part of the nodule in the medulla. The substance of a lymph node is divided into the outer cortex and the inner medulla. The cortex of a lymph node is the outer portion of the node, underneath the capsule and the subcapsular sinus. It has an outer part and a deeper part known as the paracortex. The outer cortex consists of groups of mainly inactivated B cells called follicles. When activated, these may develop into what is called a germinal centre. The deeper paracortex mainly consists of the T cells. Here the T-cells mainly interact with dendritic cells, and the reticular network is dense.

The medulla contains large blood vessels, sinuses and medullary cords that contain antibody-secreting plasma cells. There are fewer cells in the medulla.

The medullary cords are cords of lymphatic tissue, and include plasma cells, macrophages, and B cells.

Cells

In the lymphatic system a lymph node is a secondary lymphoid organ. Lymph nodes contain lymphocytes, a type of white blood cell, and are primarily made up of B cells and T cells. B cells are mainly found in the outer cortex where they are clustered together as follicular B cells in lymphoid follicles, and T cells and dendritic cells are mainly found in the paracortex.

There are fewer cells in the medulla than the cortex. The medulla contains plasma cells, as well as macrophages which are present within the medullary sinuses.

As part of the reticular network, there are follicular dendritic cells in the B cell follicle and fibroblastic reticular cells in the T cell cortex. The reticular network provides structural support and a surface for adhesion of the dendritic cells, macrophages and lymphocytes. It also allows exchange of material with blood through the high endothelial venules and provides the growth and regulatory factors necessary for activation and maturation of immune cells.

There are fewer cells in the medulla than the cortex. The medulla contains plasma cells, as well as macrophages which are present within the medullary sinuses.

As part of the reticular network, there are follicular dendritic cells in the B cell follicle and fibroblastic reticular cells in the T cell cortex. The reticular network provides structural support and a surface for adhesion of the dendritic cells, macrophages and lymphocytes. It also allows exchange of material with blood through the high endothelial venules and provides the growth and regulatory factors necessary for activation and maturation of immune cells.