Key Knowledge:
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The lymphatic system is an interconnected network of lymphatic vessels that function as a transport network for the immune response
- Phagocytic leukocytes are transported within the blood to sites of infection (recruited by chemotactic factors released from local cells)
- Monocytes will engulf the pathogens and present antigenic fragments on their surface (becoming antigen presenting cells)
- The antigen presenting cells then drain from the blood and tissues into the lymphatic system, forming a clear fluid called lymph
- The lymphatic system contains a series of filtration points (lymph nodes) that contain high amounts of lymphocytes (in lobules)
- The antigen presenting cells facilitate antigen recognition by specific T and B lymphocytes, triggering an adaptive immune response
- The filtered lymph is then returned to venous circulation (the lymphatic system uses valves to ensure the one-way movement of fluid)
Primary Lymphoid Organs
The primary lymphoid organs consist of the bone marrow and thymus, whose collective function is to produce mature lymphocytes
- The bone marrow is the site of all blood cell production (it contains multipotent hematopoietic stem cells)
- The thymus is responsible for the maturation and development of functional T lymphocytes (needed for antigen recognition)
T lymphocytes undergo both positive and negative selection in the thymus to ensure they can initiate an adaptive immune response
- Negative selection: T cells that react to the body’s own proteins (i.e. self antigens) are destroyed (to prevent autoimmunity)
- Positive selection: Only T cells that interact with MHC markers are selected (MHC markers present the foreign antigenic fragments)
Secondary Lymphoid Organs
The secondary lymphoid organs include the spleen and lymph nodes, whose collective function is initiate an adaptive immune response
- The spleen stores excess quantities of leukocytes (particularly monocytes), allowing for a more rapid response to pathogenic infection
- The lymph nodes contain high levels of lymphocytes and act as a filtration point where antigen presentation by monocytes can occur
Lymph nodes contain multiple compartments called lobules (or nodules) that are rich in immature (or inactive) B and T lymphocytes
- Antigen presenting cells within the lymph are transported to these lobules in order to interact with, and activate, the lymphocytes
The lymphatic system functions as a point of connection between the innate immune response and the adaptive immune response
- It is where the antigen presenting cells (innate system) display specific antigens to the B and T lymphocytes (adaptive system)
- In addition to its immune function, the lymphatic system also absorbs excess tissue fluid and transports fats from the small intestine