Lymph nodes: Definition

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Lymph nodes are small, bean-shaped organs that help the body fight infection. They filter lymph and trap germs, debris, and cancer cells. Lymph nodes sit throughout the body, often in groups.

Pathologists examine lymph nodes to diagnose infections, lymphomas, and cancers that have spread. This article explains what lymph nodes do, what their parts are, what conditions affect them, and what your report may say.

What are lymph nodes?

Lymph nodes are part of the lymphatic system. This system is a network of thin vessels that carries lymph through the body. The spleen and tonsils are part of the same system. Lymph flows through the nodes, which clean it before it returns to the blood.

What do lymph nodes do?

Lymph nodes protect the body in three main ways:

Filtering. Lymph nodes filter lymph, removing bacteria, waste, and other harmful material.
Storing immune cells. Lymph nodes hold white blood cells that recognize and fight infection.
Responding to threats. During an infection, a node makes more white blood cells. As a result, the node often swells.

Where are the lymph nodes in the body?

Lymph nodes sit in many parts of the body. Large groups sit in the neck, armpits, chest, abdomen, and groin. They lie near major blood vessels and near places where germs often enter the body. Each group drains lymph from a nearby region. For example, armpit nodes drain the breast and arm.

What are the parts of a lymph node?

A lymph node has several parts, each with its own job. The main parts are:

Capsule. A layer of tough connective tissue surrounds the node.
Cortex. The outer part holds round clusters of cells called follicles. Follicles contain mostly B cells, which make antibodies. Active follicles develop germinal centers, where B cells mature.
Paracortex. The area just under the cortex holds many T cells. T cells attack infected or abnormal cells.
Medulla. The center of the node holds cords of immune cells, including plasma cells.
Sinuses. These channels carry lymph through the node.
Vessels and hilum. Lymph enters through several small vessels on the outer surface. It leaves through a vessel at the hilum, an indented area on one side of the node.

How big is a normal lymph node?

Normal lymph nodes vary a lot in size. They range from about 1 millimeter to about 2.5 centimeters long. Nodes in some areas, such as the groin, are often larger than others. On a scan, doctors call enlarged nodes lymphadenopathy. An enlarged node is not always a sign of disease. Your doctor decides whether an enlarged node needs more tests.

What conditions affect lymph nodes?

Many conditions can change a lymph node. Most enlarged nodes are not cancer. The main causes are:

Infections. A node that responds to an infection can enlarge and feel tender. Pathologists call this a reactive node. See reactive lymphoid hyperplasia.
Inflammatory and immune conditions. Autoimmune diseases, such as lupus or rheumatoid arthritis, can enlarge nodes. Sarcoidosis can form small clusters of immune cells called granulomas.
Lymphoma. Lymphoma is a cancer that starts in lymphocytes. It often starts in a lymph node. The two main groups are Hodgkin lymphoma and non-Hodgkin lymphoma.
Cancer that has spread. Carcinomas and melanomas often spread to nearby nodes. Doctors call this a lymph node metastasis. Sarcomas spread to nodes less often.

How do pathologists examine lymph nodes?

Pathologists examine lymph nodes under the microscope. A doctor may remove a whole node, take a small sample with a needle, or remove nodes during cancer surgery. Fine-needle aspiration uses a thin needle to collect cells.

During cancer surgery, the surgeon often removes the nodes that drain the tumor. The pathologist finds each node, counts them, and checks each one for cancer. Sometimes the surgeon removes only the sentinel lymph node. This node is the first one that cancer cells would reach. The surgeon finds it with a tracer, a blue dye, or both. The pathologist may use immunohistochemistry to find small groups of cancer cells.

The number of nodes with cancer helps set the N category in the nodal stage. The N category is part of the TNM staging system.

What might my report say about lymph nodes?

Your report may describe lymph nodes in several standard ways. Each laboratory phrases them a little differently:

Negative for metastatic cancer. The pathologist found no cancer cells in the nodes examined.
Several positive nodes. The report lists how many nodes the pathologist examined and how many contained cancer. For example, “1 of 2 sentinel lymph nodes contains metastatic carcinoma.”
Isolated tumor cells. A cluster of fewer than 200 cancer cells, measuring 0.2 mm or less. See isolated tumor cells.
Micrometastasis. A cancer deposit larger than 0.2 mm but no larger than 2 mm.
Macrometastasis. A cancer deposit larger than 2 mm.
Extranodal extension. Cancer has grown through the capsule into the surrounding tissue. See extranodal extension.
Reactive changes. The node shows a normal immune response. It is not cancer. See reactive changes.
Sinus histiocytosis. The sinuses contain more histiocytes, a type of immune cell. It is not cancer. See sinus histiocytosis.
Matted lymph nodes. Several nodes have stuck together into one firm mass. Infection, inflammation, or cancer can cause this. See matted lymph nodes.

Questions to ask your doctor

How many lymph nodes did the pathologist examine?
Did any of my lymph nodes contain cancer?
How large was the cancer in each positive node?
Did the cancer grow outside the lymph node?
Was a sentinel lymph node removed?
Do the lymph node results change my stage?
If my nodes grew larger but contain no cancer, what caused the change?
Do I need more surgery, radiation, or other treatment because of these results?