Histology of lymphatic organs

• Cell-mediated immunity involves lymphocytes and macrophages.
• Cortex and medulla have same cell types of different proportions.
• Thymus is a central lymphoid organ located in the mediastinum.
• APC can be classified as Langerhans cells of skin, dendritic reticulum cells of lymph nodes, and microglia of central nervous system.
• APC phagocytose antigenic material and present the fragments to lymphocytes.
• Lymphocytes, T cells, B cells, and natural killer cells are types of white blood cells.
• Thymus cortical zone shows epithelial reticular cells with visible nucleoli surrounded by dark-stained T lymphocytes undergoing differentiation.
• The lymphatic nodules and germinal center are parts of the lymphoid system responsible for immunological defense of the body.
• Natural killer cells release cytokines in case of virus infection and tumors.
• Epitheloid cells are large macrophages.
• Lymph node, thymus, and spleen are capsulated lymphoid organs.
• Thymus has two main regions: cortex (deep staining) and medulla (light staining).
• Macrophages and dendritic cells are part of mononuclear phagocyte cells derived from monocytes.
• Humoral immunity involves the secretion of antibodies by B lymphocytes.
• The immune response occurs in specialized tissue and organs, particularly in lymph nodes and spleen.
• Thymus has dual origin: mesenchyme & endoderm.
• Mononuclear phagocyte system includes macrophagic cells, epitheloid cells, and antigen-presenting cells (APC).
• T cells have several roles as cytotoxic cells, helper cells, and suppressor cells.
• B cells mature to form plasma cells and secrete immunoglobulins.
• The immune response activates lymphocytes to proliferate.
• Blood-thymus barrier consists of endothelium and basal lamina of blood capillary, small amount of connective tissue with macrophages, and basal lamina of epithelial reticular cells.
• Thymus has a blood-thymus barrier that prevents the contact of antigenic material with the developing T-lymphocytes.
• Primary (central) lymphoid organs include bone marrow and thymus.
• Thymus has no nodule formation, small lymphocytes predominate, and is very active in the lymphocyte production.
• Secondary lymphoid organs include spleen and lymph nodes.
• Antigen-presenting cells (APC) are characterized with elongated ramified cell processes with low amount of lysosomes.
• The immune system consists of various organs and tissues that work together to protect the body from infection and disease.
• The characteristic histological structures of individual lymphoid organs include the lymph node, spleen, thymus, and bone marrow.
• The barrier systems in these organs include the capsule, trabeculae, and sinusoids.
• The barrier systems in these organs and their importance include the capsule, trabeculae, and sinusoids.
• Epithelial reticular cells produce four hormones: Thymosin, Thymopoietin, Thymulin, and Thymic humoral factor.
• Epithelial reticular cells are of three types: Type I epithelial reticular cells separate the cortex from the connective tissue capsule, Type II epithelial reticular cells form lymphocyte-filled compartments, and Type III epithelial reticular cells are located at the corticomedullary junction.
• Developing T cells proliferate extensively in the cortex.
• The medulla has three types of epithelial reticular cells: Type IV epithelial reticular cells are located at the corticomedullary junction and are in close association with type III cells of the cortex, Type V epithelial reticular cells form the cytoreticulum of the medulla, and Type VI epithelial reticular cells form thymic Hassal’s Corpuscules.
• Epithelial reticular cells are ectodermal and have no well-known function.
• Epithelial reticular cells isolate the cortex from the medulla and form occluding junctions by their processes.
• The primary function of the thymus is to instruct immunoincompetent T cells to achieve immunoincompetence.
• The medulla has a lesser amount of lymphocyte population and stains much lighter than the cortex.
• Blood vessels of the superficial cortex and medullary cords are non-specialized and do not allow the passage of lymphocytes.
• The convex surface of a lymph node is penetrated by many afferent lymph vessels draining into the node.