Lymph nodes are encapsulated bean-shaped lymphoid structures or tissues found throughout the body system (Figure 15); and they comprises of lymphocytes (i.e. B and T cells), macrophages and dendritic cells amongst other key immune system cells that form a mesh of immunological system. The lymph nodes generally serve as sites for the filtration of antigens from the lymph fluid or lymph prior to their presentation to immunocompetent lymphocytes and other immune system cells located within the lymph node; and the lymph nodes are richly supplied by lymphatic vessels that ensure continuous supply of lymph fluids. Lymph nodes also drain fluids from other tissues of the body especially the regional or surrounding tissue spaces aside those supplied by the lymph.
Figure 15: Schematic illustration of the lymph node. The germinal center of secondary follicle is mostly inundated with B lymphocytes.
The lymph nodes are the sites where immune responses are mounted to antigenic molecules in lymph or lymph fluids; and they are mainly located at the junctions of the lymphatic vessels all over the body. The interdigitating or cellular networks formed by phagocytic cells and dendritic cells within the lymph nodes traps particulate antigens or pathogenic microorganisms that enters the lymph nodes through the lymph; and lymphocytes swim into action to mount immunological response against the trapped antigens. The cortex, paracortex and the medulla are the three main morphological sections or parts of the lymph node. The cortex is the outermost part of the lymph nodes, and it is rich in macrophages, lymphocytes (particularly B cells) and dendritic cells. The paracortex underlie the cortex, and it is rich in T lymphocytes. Dendritic cells can also be found in the paracortex. The medulla is the innermost part of the lymph node, and it is sparingly populated with antibody-secreting plasma cells and other cells with lymphoid origin.
They trap foreign bodies or antigens in the process prior to returning the lymph or lymph fluids they contain or are made of to the systemic circulation. Lymphocytes in the lymph nodes become activated, proliferate and differentiate into numerous effector molecules following the invasion of antigens. After the interaction of the antigens with the macrophages or dendritic cells situated within the lymph nodes, the antigen is processed and presented to immunocompetent B and T cells for the instigation of appropriate immunological response against the pathogen. Lymph fluids containing antigens or pathogens are brought into the lymph node via the afferent lymph duct or lymphatics of the lymph node while the efferent lymphatics is the passageway through which immunoglobulins and lymphocytes leaves the lymph nodes to the bloodstream (Figure 15). The spleen is an oval-shaped structure that is located in the upper left abdominal cavity. It is the site where antigens or pathogens are trapped from the blood circulation, and outworn RBCs are also destroyed within the spleen.
The spleen unlike the lymph nodes primarily filters the blood that passes via it and it trap antigens in the process and present them to the lymphocytes in the peripheral lymphoid tissues. It also traps antigens from other local tissues aside those carried through the blood that passes through it. Lymphocytes and blood-borne antigens are carried into the spleen via blood vessels (e.g. the spleen arteries); and the foreign bodies are presented to the lymphocytes in the process for appropriate immunological response. Note: While the lymph nodes only trap antigens from the lymph the spleen traps antigens in the blood.
The mucosal-associated lymphoid tissues (MALT) are secondary lymphoid organs or tissues that line the mucous membranes of the GIT, urogenital tract, respiratory tract and other mucous membrane surfaces that are found in the body. The primary function of MALT is to defend the mucous membranes surfaces of the body against invasion and colonization by pathogenic microorganisms or antigens. MALT produces numerous amounts of plasma cells that produce antibodies with unique antigenic specificity. Typical examples of tissues that makeup the MALT includes Peyer’s patch (which lines the GIT) and tonsils (which are found at the base of the tongue) which express several immunological functions at the sites they are located.
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