The Lymphatic System
Previous sections have looked at the roles of various cells and molecules within the immune system. Lymphocytes are the central cell in many mechanisms. B-lymphocytes produce antibody that is important in fighting bacterial infections. Cytotoxic T-cells kill virally infected cells and T-helper cells are at the centre of several cytokine networks that coordinate and enhance the immune response. The lymphatic system is the architecture that allows these processes to occur. In order for T-helper cells to interact with B-lymphocytes they need to come into contact with them; this occurs within the lymphatic system. Prior to this interaction, the B-lymphocyte has to encounter the bacterial antigen which takes place within the peripheral lymphoid organs. Similarly T-lymphocytes first encounter viral antigens on dendritic cells in the same peripheral lymphoid organs.
Primary and Secondary Lymphoid Organs.
The lymphatic system consists of primary lymphoid organs, secondary lymphoid organs and lymphatic vessels that link these organs to the bloodstream.
The primary lymphoid organs (central lymphoid organs) are where lymphocytes arise and are the bone marrow and the thymus. The secondary lymphoid organs (peripheral lymphoid organs) contain high numbers of lymphocytes and non-lymphoid tissue. They are lymph nodes, the spleen and the mucosal-associated lymphoid tissue (MALT). The most important of the mucosal-associated lymphoid tissues is the gut-associated lymphoid tissue (GALT) that includes the tonsils and adenoids and most significantly Peyer’s patches of the small intestine. The GALT is so important because the gastro-intestinal tract is a major route of entry into the body for pathogenic organisms. It is in secondary lymphoid organs that lymphocytes encounter antigen and the lymphocytes are constantly circulated between these organs and the bloodstream to enable these encounters to occur.
The lymphatic vessels drain extra-cellular fluid produced by osmostic movement from the capillaries into the tissues. This lymph fluid drains through lymph nodes into the thoracic duct where is empties into the left subclavian vein. Infection can occur anywhere in the body, but this circulation of fluid brings the antigens of invading organisms into contact with naïve lymphocytes in order to activate them.
Lymph Nodes
All of the secondary lymphoid organs are functionally identical, i.e. their role is to enable lymphocytes to encounter antigen. However they are structurally very different. A simplistic diagram of a lymph node is shown below.
It is a highly organised structure with distinct B-cell and T-cell areas. The B-ells are located in follicles whilst the T-cells are more diffusely located within the paracortical regions. This organisation into areas is vital in enabling the interaction between B- and T-cells that is necessary for an effective immune response. Within a follicle germinal centres are seen. These are sighst of massive B-cell proliferation in response to stimulation by an antigen and a helper T-cell.
After proliferating and differentiating within the lymph node, effector lymphocytes (activated cytotoxic T-cells or plasma cells) migrate out of the lymph node via the efferent vessel. They enter the blood stream and subsequently return to the site of infection.
