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Lymphatic System

Describes lymph, lymph organs and vessels, and lymphocytes.

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Lymphatic System

Do you still have your tonsils?

Many people have their tonsils removed because they can hold on to germs that cause throat infections. Although sometimes they can do more harm than good, tonsils are there to help you. They trap pathogens so that they can be destroyed before they enter farther into your body.

The Lymphatic System and the Immune Response

If pathogens get through the body’s first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. It is called an immune response, and is a specific type of response. The immune system has a special response for each type of pathogen.

The immune system (Figure below) is also part of the lymphatic system—named for lymphocytes, which are the type of white blood cells involved in an immune response. They include several lymph organs, lymph vessels, lymph, and lymph nodes.

This diagram illustrates the parts of the immune system

This diagram shows the parts of the immune system. The immune system includes several organs and a system of vessels that carry lymph. Lymph nodes are located along the lymph vessels.

Lymph Organs

The lymph organs are the red bone marrow, tonsils, spleen, and thymus gland. They are described below (Figure below).

Each lymph organ has a different job in the immune system

Each lymph organ has a different job in the immune system.

Lymph and Lymph Vessels

Lymph vessels make up a circulatory system that is similar to the cardiovascular system, which you can read about in a previous concept. Lymph vessels are like blood vessels, except they move lymph instead of blood. Lymph is a yellowish liquid that leaks out of tiny blood vessels into spaces between cells in tissues. Where there is more inflammation, there is usually more lymph in tissues. This lymph may contain many pathogens.

The lymph that collects in tissues slowly passes into tiny lymph vessels. It then travels from smaller to larger lymph vessels. Lymph is not pumped through lymph vessels like blood is pumped through blood vessels by the heart. Instead, muscles around the lymph vessels contract and squeeze the lymph through the vessels. The lymph vessels also contract to help move the lymph along. The lymph finally reaches the main lymph vessels in the chest. Here, the lymph drains into two large veins. This is how the lymph returns to the bloodstream.

Before lymph reaches the bloodstream, pathogens are removed from it at lymph nodes. Lymph nodes are small, oval structures located along the lymph vessels. They act like filters. Any pathogens filtered out of the lymph at lymph nodes are destroyed by lymphocytes in the nodes.


Lymphocytes (Figure below), a type of white blood cell, are the key cells of an immune response. There are trillions of lymphocytes in the human body. They make up about one quarter of all white blood cells. Usually, fewer than half of the body’s lymphocytes are in the blood. The rest are in the lymph, lymph nodes, and lymph organs.

Lymphocytes are the key cells of an immune response

This image of a lymphocyte was made with an electron microscope. The lymphocyte is shown 10,000 times its actual size.

There are two main types of lymphocytes:

  1. B cells.
  2. T cells.

Both types of lymphocytes are produced in the red bone marrow. They are named for the sites where they grow larger. The "B" in B cells stands for “bone.” B cells grow larger in red bone marrow. The "T" in T cells stands for “thymus.” T cells mature in the thymus gland.

B and T cells must be “switched on” in order to fight a specific pathogen. Once this happens, they produce an army of cells ready to fight that particular pathogen. How can B and T cells recognize specific pathogens? Pathogens have proteins, often located on their cell surface. These proteins are called antigens. An antigen is any protein that causes an immune response, because it is unlike any protein that the body makes. Antigens are found on bacteria, viruses, and other pathogens. Your body sees these as foreign, meaning they do not belong in your body.


  • The immune system includes lymph organs, lymph vessels, lymph, and lymph nodes.
  • Lymph organs include the red bone marrow, thymus gland, spleen, and tonsils.
  • Lymphocytes, including B cells and T cells, are key cells in the immune response.

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Use the resource below to answer the questions that follow.

  1. What is the main function of the lymphatic system?
  2. What are the tonsils?
  3. How do the lymph nodes act in the lymphatic system?


  1. What are three examples of lymph organs?
  2. What is the role of the tonsils?
  3. What are lymph nodes? What is their function?
  4. Distinguish between B and T lymphocytes.

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antigen Protein that is recognized as foreign and causes an immune response.
bone marrow Soft connective tissue in spongy bone; makes blood cells.
immune response Body's defense against a specific pathogen.
immune system Organ system that defends the body from pathogens.
lymph Clear fluid that drains through the lymphatic system and into the bloodstream.
lymph nodes Small, oval structures located along the lymph vessels where pathogens are filtered out and destroyed.
lymphocytes Type of white blood cell that is a key cell in the immune response to a specific pathogen.
spleen Organ that filters pathogens out of the blood.
thymus gland Gland that stores and matures lymphocytes.
tonsils Glands that trap pathogens that enter the body through the mouth or nose.

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