Lymphoid Tissue Forms the basis of the immune system of the body Forms the basis of the immune system of the body Lymphocyte, the principal cell of lymphoid.

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  • Lymphoid TissueForms the basis of the immune system of the bodyLymphocyte, the principal cell of lymphoid tissue, is responsible for the proper functioning of the immune systemClassified as primary & secondary lymphoid tissueOrganized into diffuse and nodular lymphoid tissue

  • Types of Lymphoid OrgansPrimary lymphoid organs are responsible for the development and maturation of lymphocytes into mature cells e.g. fetal liver, bone marrow and thymusSecondary lymphoid organs are responsible for the proper environment in which the immunocompetent cells can react with each other and with the antigens e.g. lymph nodes, spleen, and mucosa associated lymphoid tissue

  • Organization of Lymphoid Tissue1. Diffuse Lymphoid Tissue

    Occurs throughout the body, especially under wet epithelial membranes where the connective tissue is infiltrated by lymphoid cells e.g. lymphocytes, plasma cells, macrophages and reticular cells Particularly evident in the lamina propria of GIT & subepithelial connective tissue of the respiratory tract

  • 2. Nodular Lymphoid TissueIncludes:Lymph nodesThymusSpleenTonsils

  • Cells of Immune SystemLymphocytes are generally classified into:B lymphocytes (B cells)T lymphocytes (T cells)Null cells: composed of two typesStem cellsNatural killer cells

  • B LymphocytesSmall lymphocytes that are formed, and become immunocompetent in the bone marrow.The activated cells enter the general circulation, undergo mitosis, and transform into antibody-producing plasma cells and B-memory cells

  • T Lymphocytes

    Histologically identical to B lymphocytesFormed in bone marrow, migrate to the cortex of the thymus to become immunocompetentThere are several subtypes e.g. T helper cells, T cytotoxic cells, T suppressor cells and T memory cellsAssist B lymphocytes in the performance of their functions

  • Antigen Presenting Cells

    Derived from monocytesThey phagocytose, catabolize and process antigen and present to T lymphocytesInclude macrophages, dendritic cells (Langerhans cells of the epithelium & mucosa), B cells and epithelial reticular cells of thymus

  • Stem CellsUndifferentiated cellsGive rise to the various cellular elements of the blood

  • Natural Killer CellsCytotoxic cellsFormed in the bone marrowBecome immunocompetent as soon as they are formedThey kill some virally altered cells and tumor cells

  • Lymph NodesOvoid, kidney shaped organs Each node has:A convex surface which receives afferent lymph vesselsA hilum where efferent lymph vessels leave and drain lymph from the organs

  • Histological Structure of Lymph NodeEach lymph node has a dense irregular collagenous connective tissue capsule Derived from the capsule, connective tissue septae (trabeculae) extend into the outer part (cortex) of the node and divide it into incomplete compartmentsThe framework of the node is formed by reticular connective tissue

  • Each lymph node is divided into three regions: Cortex Paracortex Medulla


  • Lymph Nodes: CORTEXcontains the:Subcapsular sinus, which is continuous and drains into the Cortical sinusesLymphoid nodules (primary & secondary) composed mainly of B lymphocytes and reticular cellsCPCMncscscsn

  • Lymph Nodes: PARACORTEXIt is the region between cortex and medullaContains thymus dependent zone mainly T lymphocytesHigh endothelial venules ( HEV) lined by simple cuboidal epithelium instead of simple squmaus epithelium to facilitate passage of mature T lymphocyte coming from thymus to paracortex of lymph node. Has NO nodules


  • Lymph Nodes: MEDULLA consists of: Medullary cords Medullary sinusoides.The medullary cords are composed of T lymphocytes, plasma cells and macrophagesThe medullary sinusoides are continuous with the subcapsular and cortical sinuses.There in no lymphatic Nodules.MSpc

  • Lymph NodulesLymph nodules are small masses of lymph tissue in which B lymphocytes are produced.

    Lymph nodules may be:Primary nodulesSecondary nodules PNSN

  • Primary noduleswhich are formed of virgin (not activated) and memory B lymphocytes

  • Secondary NoduleContains:Germinal centres, central light areas filled with immature lymphocytes. Here new lymphocytes proliferate from stem cells that originate in the bone marrow. Germinal centers contains large lymphocytes, mitotic figures, macrophages and plasma cellsThe germinal centre is surrounded by a darker-staining region called the corona that contains small B lymphocytes ( not activated).c

  • Lymph Flow Through The Lymph NodeAfferent lymph vessels

    Subcapsular sinuses Cortical sinuses

    Medullary sinuses

    Efferent lymphatic vessels

  • Lymph Node: FunctionMaintenance and production of immunocompetent cells Proliferation of lymphocytes. Activated T- and B- lymphocytes multiply in the lymph nodesFiltration of lymph ( not blood)Phagocytosis

  • ThymusBilobed lymphoid organ located in thoraxDerived from endodermEnclosed in a thin connective tissue capsuleSeptae extend from the capsule into the organ, incompletely subdividing it into lobulesPossesses no lymph nodules LLSC

  • Thymus contdEach lobule is divided into an outer cortex and inner medullaCORTEX: Composed of epithelial reticular cells, macrophages, and thymocytes (small T lymphocytes). Here the immature T cells undergo proliferation, reticular cells release hormone and factor to activate T lymphocyte and transform into immunocompetent cells and then migrate to medulla MEDULLA: consists of epithelial reticular cells, large T lymphocytes and thymic (Hassalls) corpuscles Microphage will engulf not activated T lymphocyte.

    1 - cortex 2 - medulla 3 - Hassal's corpuscle 4 - interlobular connective tissue (septa)

  • Hassalls CorpuscleAre composed of groups of keratinized epithelial cellsIncrease in number with ageProbably represent a degenerative phenomenon


  • Epithelial Reticular CellsSeparate the cortex from the capsule and the trabeculaeSurround the cortical capillaries thus forming a blood-thymus barrierForm a specialized barrier between cortex & medulla, preventing medullary material from gaining access to the cortexThus provide an antigen free environment for the potentiation of the immunocompetent T lymphocytesThey also form Hassalls (thymic) corpuscles

  • Thymus

  • Thymus

  • Lymph Node

  • Lymph Node


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