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Organs of the Immune System

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The human immune system is made up of several different organs and cells that work with each other to protect the body from bacterial, parasitic, fungal, and viral infections. These organs and cells are highly specialized, and our understanding of them and their functions may in turn give us a better understanding of what happens when something goes wrong.


The organs of the immune system serve two functions:

  1. Create the cells that are involved in immune response. These are classified as primary lymphoid organs.
  2. Operate as locations for immune function. These are the secondary lymphoid organs.

Primary Lymphoid Organs

Bone Marrow: This soft tissue located in the center of the bones serves the main function of producing B cells, natural killer cells, granulocytes and immature thymocytes, in addition to red blood cells and platelets. Thymus: Located in the thoracic cavity near the heart, the thymus matures the T cells into thymocytes. Immature thymocytes, also called prothymocytes, leave the bone marrow and travel into the thymus where they undergo a significant maturation process. It is here that beneficial T cells are separated from T cells that have the potential to produce a negative autoimmune response. These are destroyed while the mature T cells are released into the lymph system.

Secondary Lymphoid Organs

Spleen: The spleen is composed of:

The spleen's main task is to filter the blood of foreign materials called antigens. The spleen achieves this in two ways:

  1. When blood passes through the spleen, it filters out the antigens from the bloodstream.
  2. The spleen sends traveling macrophages and dendritic cells into the bloodstream to capture antigens and return them to the spleen.

The antigens are then held in the spleen where the appropriate B cells or T cells decide their fate. In a healthy immune system, this means elimination. The spleen is also the location where B cells activate and produce antibodies. Old red blood cells are sent to the spleen to die.

Lymph Nodes: Bean-shaped and located throughout the body, lymph nodes serve to filter lymph, or bodily fluids. Made up of mostly T cells, B cells, dendritic cells and macrophages, the lymph nodes function similarly to the spleen. They drain the fluid of most bodily tissues, sorting out antigens before returning the lymph to circulation. Also similar to the spleen, the lymph nodes are another location where the dendritic cells and macrophages hand these antigens over to the B cells and T cells which then give the necessary immune response.


Photo Credit: Microbe World

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