The Lymphatic System

Published: November 11, 2016
Last reviewed: April 29, 2017

What is the lymphatic system?

The lymphatic system is a network of organs rich in the immune cells called lymphocytes. It includes:

  • The lymph nodes, lymphatic vessels and the lymph, which flows through them
  • The spleen
  • The tonsils
  • The lymphoid tissue in the small intestine (Peyer’s patches)
  • The thymus (in children)
  • The bone marrow

Primary Lymphoid Organs

The primary or central lymphoid organs produce lymphocytes from immature progenitor cells. They include the bone marrow and thymus.

Bone Marrow

The bone marrow produces lymphocytes B and T from immature progenitor cells.

Lymphocytes B produced in the bone marrow enter the blood and from there the secondary lymphoid organs: lymph nodes, spleen, tonsils and the lymphoid tissue in the small intestine.

Lymphocytes T travel to the thymus where they develop further.


The thymus develops soon after birth and disappears after puberty. It is located behind the breastbone in the upper chest. In the thymus, the immature T lymphocytes that have come from the bone marrow mature further.

Secondary Lymphoid Organs

Secondary or peripheral lymphoid organs maintain mature lymphocytes. They include the lymph nodes, spleen, tonsils, the lymphoid tissue in the small intestine and appendix and other mucosa-associated lymphoid tissues (MALT). They are positioned in the places where they can get in contact with antigens (microbes, foreign particles, cancer cells). Antigens activate the B lymphocytes and turn them into the plasma cells, which produce specific antibodies against these antigens.

Mature B lymphocytes recirculate between the blood and secondary lymphoid organs until they are activated by specific antigens.

Lymph Nodes

The lymph nodes are bean-like organs found mainly in the chest and abdominal cavity, neck, armpits and groin. They remove microbes, foreign particles and cancerous cells from the lymph, which flows through them.


The spleen lies in the upper left abdomen. It removes microbes, foreign particles and damaged blood cells from the blood.

Tonsils and Adenoids

The tonsils lie between the mouth and throat. They can catch and destroy some of the microbes and foreign particles that come into the mouth by air and food.

The adenoids lie behind the nasal cavity. They catch and destroy some of the inhaled microbes and foreign particles.

Peyer’s Patches in the Small Intestine

The Peyer’s patches are found in the mucosal layer of the small intestine, mainly in the ileum. They can catch and destroy some microbes and foreign particles ingested by food.

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