Lymphoid Tissue
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Lymphoid Tissue

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Questions and Answers

What is the primary site of origin for precursor lymphoblasts?

  • Lymph nodes
  • Thymus
  • Bone marrow (correct)
  • Spleen

    • Which structure is primarily responsible for T cell maturation and differentiation?

  • Bone marrow
  • Lymph nodes
  • Thymus (correct)
  • Spleen

    • What are Hassal corpuscles primarily associated with?

  • Bone marrow
  • Medulla of the spleen
  • Lymph nodes
  • Cortex of the thymus (correct)

    • Which of the following statements about secondary lymphoid organs is true?

    <p>They contain lymphoid nodules.</p> Signup and view all the answers

    Which description accurately depicts the thymus?

    <p>It is a bilobed structure that involutes in adulthood.</p> Signup and view all the answers

    What characteristic distinguishes innate immune response from adaptive immune response?

    <p>Remains the same magnitude with every encounter</p> Signup and view all the answers

    Which of the following describes a function of lymphoid organs?

    <p>Filter and trap pathogens</p> Signup and view all the answers

    Which cells are considered first responders to pathogens in the innate immune response?

    <p>Granulocytes</p> Signup and view all the answers

    What is an example of a physical barrier in the innate immune response?

    <p>Skin</p> Signup and view all the answers

    Which lymphoid tissue is primarily associated with the gastrointestinal tract?

    <p>Mucosa-associated lymphoid tissue (MALT)</p> Signup and view all the answers

    Which characteristic does NOT describe the innate immune response?

    <p>Specificity for individual pathogens</p> Signup and view all the answers

    Which organ is NOT classified as a lymphoid organ?

    <p>Liver</p> Signup and view all the answers

    What role do natural killer (NK) cells play in the innate immune response?

    <p>Identifying and destroying infected or abnormal cells</p> Signup and view all the answers

    What type of cells predominantly populate the cortex of the thymus?

    <p>T lymphoblasts</p> Signup and view all the answers

    What is a significant structural change that occurs in the thymus during involution at puberty?

    <p>Replacement of tissue by fat cells</p> Signup and view all the answers

    Negative selection of T cells occurs in which region of the thymus?

    <p>Medulla</p> Signup and view all the answers

    What is the role of the Aire gene in the thymus?

    <p>Promotes presentation of tissue-specific antigens</p> Signup and view all the answers

    Which cells form the blood-thymus barrier?

    <p>Epithelial reticular cells and endothelial cells</p> Signup and view all the answers

    What does survival of T cells during negative selection depend on?

    <p>Failure to recognize self-antigens</p> Signup and view all the answers

    What is the main function of cortical epithelial cells in the thymus?

    <p>Support and differentiate developing T cells</p> Signup and view all the answers

    What characterizes the appearance of the thymus during involution?

    <p>Predominantly fat cells with sparse lymphocytes</p> Signup and view all the answers

    What are the main components found in the medullary cords of the spleen?

    <p>T and B lymphocytes and plasma cells</p> Signup and view all the answers

    What distinguishes the medullary sinuses in the spleen?

    <p>They separate the medullary cords and contain many macrophages</p> Signup and view all the answers

    What gives the white pulp of the spleen its characteristic appearance?

    <p>A variety of lymphoid nodules surrounded by red pulp</p> Signup and view all the answers

    What is the primary role of the lymphatic system?

    <p>To transport interstitial fluid back to blood circulation</p> Signup and view all the answers

    What is the function of the reticular cells found in the spleen?

    <p>They promote lymphocyte proliferation and organization</p> Signup and view all the answers

    Why is the central arteriole important in the functioning of the white pulp?

    <p>It supplies blood to lymphoid follicles</p> Signup and view all the answers

    Which organ is NOT classified as a primary lymphoid organ?

    <p>Lymph node</p> Signup and view all the answers

    What is a defining characteristic of splenic cords?

    <p>They are branched, cord-like masses of lymphoid tissue</p> Signup and view all the answers

    What type of lymphocytes are matured in the thymus?

    <p>T-lymphocytes</p> Signup and view all the answers

    Which statement about the red pulp of the spleen is true?

    <p>It is involved in the filtration of blood</p> Signup and view all the answers

    Which of the following is considered a secondary lymphoid organ?

    <p>Spleen</p> Signup and view all the answers

    How can metastatic cancer affect lymph nodes?

    <p>It can lead to the formation of secondary tumors</p> Signup and view all the answers

    What is the function of leukocytes in the lymphoid organs?

    <p>To eliminate antigens</p> Signup and view all the answers

    Where does differentiation of T-lymphoblasts into T-lymphocytes occur?

    <p>Thymus</p> Signup and view all the answers

    Which cells are considered motile in lymphoid organs?

    <p>Lymphocytes</p> Signup and view all the answers

    Which of the following is primarily involved in the activation and proliferation of lymphocytes?

    <p>Secondary lymphoid organs</p> Signup and view all the answers

    What structure comprises mucosa-associated lymphoid tissues (MALT)?

    <p>Tonsils and appendix</p> Signup and view all the answers

    Which of the following best describes the role of primary lymphoid organs?

    <p>They give rise to lymphocytes after maturation.</p> Signup and view all the answers

    What is the primary function of the pharyngeal tonsil?

    <p>Regulate adaptive immunity</p> Signup and view all the answers

    Which of the following organs is classified as a primary lymphoid organ?

    <p>Bone marrow</p> Signup and view all the answers

    What type of cells are primarily responsible for lymphocyte activation and proliferation in secondary lymphoid organs?

    <p>Lymphocytes</p> Signup and view all the answers

    How do primary lymphoid organs differ from secondary lymphoid organs?

    <p>They are responsible for lymphocyte development</p> Signup and view all the answers

    Which of the following motile cells is NOT found in lymphoid organs?

    <p>Neutrophils</p> Signup and view all the answers

    What is the primary function of lymph nodes?

    <p>Filter lymph</p> Signup and view all the answers

    Which structures enter and exit the lymph nodes?

    <p>Afferent and efferent lymphatic vessels</p> Signup and view all the answers

    Which region of the lymph node is primarily responsible for antigen presentation?

    <p>Cortex</p> Signup and view all the answers

    What encapsulates a lymph node?

    <p>Dense connective tissue</p> Signup and view all the answers

    What do the trabeculae in a lymph node do?

    <p>Extend within the lymph node parenchyma</p> Signup and view all the answers

    What type of surface does a lymph node have where afferent lymphatic vessels enter?

    <p>Convex surface</p> Signup and view all the answers

    Which of the following sinuses does lymph drain through in a lymph node?

    <p>Subcapsular sinus to trabecular sinus to medullary sinus</p> Signup and view all the answers

    What is the size range of lymph nodes?

    <p>10mm to 2cm</p> Signup and view all the answers

    What is the primary function of B lymphocytes in the adaptive immune response?

    <p>To differentiate into plasma cells that secrete antibodies</p> Signup and view all the answers

    Which statement accurately describes the adaptive immune response?

    <p>It becomes faster with subsequent exposures to the same pathogen</p> Signup and view all the answers

    What is the role of major histocompatibility complex (MHC) in the cell-mediated immune response?

    <p>To present antigens to T lymphocytes</p> Signup and view all the answers

    Which type of immunity is primarily attributed to B lymphocytes?

    <p>Humoral immunity</p> Signup and view all the answers

    What are memory cells responsible for in the adaptive immune response?

    <p>Facilitating a quicker response to subsequent infections</p> Signup and view all the answers

    Where do B lymphocytes mature?

    <p>Bone marrow</p> Signup and view all the answers

    What types of antibodies are produced by plasma cells?

    <p>IgA, IgE, IgM, IgG, and IgD</p> Signup and view all the answers

    Which lymphocytes are primarily involved in the cell-mediated immune response?

    <p>T lymphocytes</p> Signup and view all the answers

    What are Stave Cells primarily associated with?

    <p>The lining of splenic sinusoids</p> Signup and view all the answers

    What is the function of Stave Cells in the spleen?

    <p>They facilitate the passage of lymphocytes and platelets into sinusoids.</p> Signup and view all the answers

    What specific structure allows blood to flow through the splenic red pulp?

    <p>Open circulation</p> Signup and view all the answers

    What characteristic of Stave Cells is notable regarding their basal lamina?

    <p>It is highly discontinuous and allows the passage of RBCs.</p> Signup and view all the answers

    How are Stave Cells oriented in relation to blood flow?

    <p>Parallel to the direction of blood flow</p> Signup and view all the answers

    What is a distinctive feature of the nuclei in Stave Cells?

    <p>They are large and bulging into the sinusoidal lumen.</p> Signup and view all the answers

    Which component is NOT directly related to the function of Stave Cells?

    <p>Hormones</p> Signup and view all the answers

    Which structure is lined by Stave Cells?

    <p>Splenic sinusoids</p> Signup and view all the answers

    What distinguishes the thymus from other lymphoid organs?

    <p>Absence of lymphoid nodules</p> Signup and view all the answers

    Which structure lacks tonsillar crypts?

    <p>Palatine tonsil</p> Signup and view all the answers

    Where are high endothelial venules primarily located?

    <p>Paracortex of the lymph node</p> Signup and view all the answers

    What is unique about Peyer’s patches compared to the spleen?

    <p>Associated with the gastrointestinal tract</p> Signup and view all the answers

    Which feature is NOT characteristic of the spleen?

    <p>Presence of Hassal corpuscles</p> Signup and view all the answers

    What feature is missing in the thymus compared to secondary lymphoid organs?

    <p>Lymphoid nodules</p> Signup and view all the answers

    Which lymphoid tissue is characterized by the presence of a capsule?

    <p>Lymph node</p> Signup and view all the answers

    What distinguishes secondary lymphoid organs from primary ones?

    <p>Presence of lymphoid nodules</p> Signup and view all the answers

    Which lymphoid organ lacks lymphoid nodules?

    <p>Thymus</p> Signup and view all the answers

    What unique feature is associated only with MALT?

    <p>M Cells</p> Signup and view all the answers

    Which of the following accurately describes the arrangement of lymphatic vessels in the lymph nodes?

    <p>Present in both afferent and efferent configurations</p> Signup and view all the answers

    Which feature is unique to the spleen compared to other lymphoid organs?

    <p>White pulp containing a central artery</p> Signup and view all the answers

    What is true about the cortex and medulla structures in the thymus compared to other lymphoid organs?

    <p>The thymus uniquely possesses a cortex and medulla arrangement</p> Signup and view all the answers

    What is the primary role of antibodies produced by plasma cells?

    <p>To neutralize and mark pathogens for destruction</p> Signup and view all the answers

    Which statement accurately describes a characteristic of cell-mediated immunity?

    <p>It is primarily mediated by T lymphocytes</p> Signup and view all the answers

    What happens to the response of the adaptive immune system upon subsequent exposure to the same pathogen?

    <p>The response becomes faster and more robust</p> Signup and view all the answers

    Which type of immune response is specifically mediated by B lymphocytes?

    <p>Humoral immunity</p> Signup and view all the answers

    Which type of lymphocyte is primarily associated with the elimination of antigens?

    <p>T lymphocytes</p> Signup and view all the answers

    Why are memory cells important in the adaptive immune response?

    <p>They ensure a faster response upon re-exposure to a pathogen</p> Signup and view all the answers

    What distinguishes adaptive immunity from innate immunity?

    <p>Adaptive immunity is specific to particular pathogens</p> Signup and view all the answers

    What constitutes the essential liquid component that carries antibodies and lymphocytes within lymphoid organs?

    <p>Lymph</p> Signup and view all the answers

    What distinguishes the cortex of the thymus from the medulla in terms of cellular composition?

    <p>The cortex has a higher density of lymphoblasts compared to the medulla.</p> Signup and view all the answers

    In patients with DiGeorge syndrome, what is a primary aberration in thymic development?

    <p>Underdevelopment or absence of the thymus.</p> Signup and view all the answers

    Which structural feature of the thymus aids in the organization of its lobules?

    <p>Trabeculae or septa.</p> Signup and view all the answers

    What primarily causes the reduction in lymphoid tissue mass within the thymus during adulthood?

    <p>Accumulation of adipose tissue within the thymus.</p> Signup and view all the answers

    Which statement best describes the staining characteristics of the thymus?

    <p>The cortex stains deeper due to more lymphoblasts.</p> Signup and view all the answers

    What is the fate of T lymphoblasts after they mature in the thymus?

    <p>They migrate to the peripheral blood.</p> Signup and view all the answers

    Which tissue surrounds and encapsulates the thymus, aiding its structural integrity?

    <p>Connective tissue.</p> Signup and view all the answers

    What is one of the consequences of thymic aplasia in DiGeorge syndrome patients?

    <p>Severely impaired cell-mediated immunity.</p> Signup and view all the answers

    What may occur if developing T lymphoblasts are exposed to antigens in the blood?

    <p>They may trigger an autoimmune response.</p> Signup and view all the answers

    Which of the following describes a function of Hassall corpuscles in the thymus medulla?

    <p>They regulate dendritic cell activity in the thymus.</p> Signup and view all the answers

    What is the primary factor that allows T lymphoblasts to survive during the positive selection process?

    <p>Their ability to bind to MHC molecules.</p> Signup and view all the answers

    Which statement accurately reflects the characteristics of the thymus medulla compared to the cortex?

    <p>The medulla has light staining due to fewer lymphocytes.</p> Signup and view all the answers

    What is the consequence of selecting for non-self-reactive T cells in the thymus?

    <p>Promotion of peripheral tolerance.</p> Signup and view all the answers

    What cellular components predominantly make up splenic nodules in the white pulp?

    <p>B-lymphocytes with germinal centers</p> Signup and view all the answers

    Why is the term 'central arteriole' considered a misnomer in the context of the white pulp?

    <p>It is located at the periphery due to lymphocyte proliferation.</p> Signup and view all the answers

    What is the final pathway of blood flow from central arterioles in the white pulp?

    <p>Through closed circulation or open circulation</p> Signup and view all the answers

    How do central arterioles transition as they branch within the white pulp?

    <p>They eventually lose their sheath and become penicillar arterioles.</p> Signup and view all the answers

    What surrounds capillaries as they emerge from penicillar arterioles in the spleen's white pulp?

    <p>Macrophages</p> Signup and view all the answers

    What is indicated by the presence of a germinal center in splenic nodules?

    <p>B-lymphocytes are undergoing activation and differentiation.</p> Signup and view all the answers

    What originates from the splenic artery before branching into the white pulp?

    <p>Trabecular arteries</p> Signup and view all the answers

    What characterizes the blood flow from the white pulp to the red pulp in the spleen?

    <p>It includes a transition through capillaries.</p> Signup and view all the answers

    Study Notes

    Immune System

    • The immune system is comprised of the innate and adaptive immune responses (types of immune responses), and cells of the adaptive immune response.
    • The lymphatic system contains lymph vessels which transport interstitial fluid back to the blood stream and the lymphoid organs.
    • Lymphoid organs are either primary, secondary, or motile.
      • Primary lymphoid organs give rise to lymphocytes.
      • Secondary lymphoid organs allow lymphocytes to activate and proliferate.
      • Motile cells in lymphoid organs include leukocytes, lymphocytes, macrophages, and dendritic cells.

    Primary Lymphoid Organs

    • Bone marrow:
      • Site of maturation of B-lymphocytes and NK cells.
      • Origin for precursor lymphoblasts for myeloid and lymphoid stem cells.
    • Thymus:
      • Site of maturation of T-Lymphocytes.
      • The thymus has a cortex and medulla.
      • The cortex is densely populated by T lymphoblasts and lymphocytes.
      • The medulla contains T-Lymphocytes, B-lymphocytes, and plasma cells.

    Secondary Lymphoid Organs

    • Lymph nodes:
      • Present throughout the body
      • Bean shaped
      • Presence of lymphoid nodules and lymphocytes.
    • Spleen:
      • Has a white pulp and red pulp.
      • The white pulp appears white in freshly cut specimens and is surrounded by the red pulp.
      • The red pulp is comprised of splenic cords and splenic sinusoids (spaces in between).
      • The white pulp is composed of lymphoid tissue, which is filled with immune cells and red blood cells.
      • Lymphocytes and white blood cells are responsible for immunity.
      • The spleen is also responsible for filtering blood.
      • It is located in the upper left abdomen.
      • White pulp:
        • Splenic/lymphoid nodule
          • Central arteriole
          • PALS (Periarteriolar Lymphoid Sheaths)
      • Red pulp:
        • Splenic cords
        • Splenic sinusoids
          • Stave cells
    • Mucosa-associated lymphoid tissue (MALT):
      • Includes tonsils and appendix
      • Contains large numbers of lymphocytes and macrophages.
      • The mucosa is a layer of tissue that lines the inner surfaces of the body, such as the nose, throat, and digestive tract.
      • MALT is an important part of the immune system and is responsible for protecting these surfaces from infection.

    T cell Maturation within the Thymus

    • Positive Selection:
      • Occurs in the thymic cortex.
      • T cells must be able to recognize MHC (major histocompatibility complex) molecules. - The MHC is found in the thymic cortex on epithelial cells that line the blood vessels.
      • T cells that recognize self-MHC molecules are kept and continue to mature.
    • Negative Selection:
      • Occurs in the thymic medulla.
      • T cells that recognize self-antigens are removed.
      • The Aire gene (autoimmune regulator) expressed in the medulla allows for the presentation of self-antigens to developing T cells.
      • T cells that recognize these self-antigens are eliminated.
      • This process ensures that T cells do not attack the body's own cells.
    • Blood-Thymus Barrier:
      • Protects developing T cells from antigens in the blood stream.
      • Formed by the epithelium of the thymus, which forms sheaths around the blood vessels.
      • It consists of epithelial reticular cells and endothelial cells joined by tight junctions.

    Lymphatic System

    • The lymphatic system is made up of a network of lymphatic vessels that transport interstitial fluid (fluid found between cells) back to the blood circulation.
    • Lymphoid organs house the lymphocytes and other immune cells.
    • ** sentinel lymph node:**
      • Cancer cells can spread to this lymph node.

    Lymphoid Organs

    • Lymphoid Nodules (Lymphatic nodules):
      • Contain T and B lymphocytes.
      • Can be found in the lymph nodes, spleen, and MALT.
      • They are clusters of lymphocytes, macrophages, and dendritic cells.
    • Hassal's corpuscles:
      • Are found in the medulla of the thymus.
      • They are structures that are thought to play a role in the development and regulation of T cells.

    Medical Applications

    • Metastatic Cancer Cells:
      • Can spread to lymph nodes, especially the sentinel lymph node.
      • Cancer cells in lymph nodes may lead to the formation of secondary tumors.
    • Immune Deficiency:
      • Can be caused by a lack of or malfunction of certain immune cells, such as T cells or B cells.
      • These deficiencies can make people more susceptible to infections.
    • Autoimmune Disorders:
      • Occur when the immune system attacks the body's own cells and tissues.
      • These disorders can affect any part of the body.

    Adaptive Immune Response

    • The body's specific defense mechanism against pathogens.
    • Faster and more efficient with subsequent exposure to the same pathogen.
    • Both B and T lymphocytes play a crucial role.
    • Two main types: Humoral and Cell-Mediated.

    Humoral Immune Response

    • Mediated by B lymphocytes, which transform into plasma cells.
    • Plasma cells produce antibodies (IgA, IgE, IgM, IgG, IgD).

    Cell-Mediated Immune Response

    • T lymphocytes, guided by MHC (Major Histocompatibility Complex), eliminate antigens.

    Lymphoid Organs

    • Primary Lymphoid Organs: responsible for lymphocyte development (e.g., bone marrow, thymus).
    • Secondary Lymphoid Organs: sites for lymphocyte activation and proliferation (e.g., lymph nodes, spleen).
    • Motile Cells within Lymphoid Organs: Leukocytes, Lymphocytes, Macrophages, Dendritic Cells.

    Lymph Nodes

    • Bean-shaped, encapsulated structures.
    • 10mm to 2cm in size.
    • Strategically placed along lymphatic vessels.
    • Possess afferent and efferent lymphatic vessels, a unique characteristic.
    • Primary function: filtration of lymph.
    • Prevent the spread of antigens and tumor cells.
    • Sites of antigen presentation to trigger antibody production by plasma cells.

    Regions of a Lymph Node

    • Cortex: Outer layer, rich in lymphocytes, particularly B cells.
    • Paracortex: Region between the cortex and medulla, populated by T cells and dendritic cells.
    • Medulla: Innermost layer, contains medullary cords (B cells, plasma cells, macrophages) and medullary sinuses (lymph drainage).

    Spleen

    • Largest lymphoid organ in the body.
    • Found in the left upper quadrant of the abdomen.
    • Filters blood and removes aged red blood cells.
    • Stores white blood cells and platelets.
    • Houses both white pulp and red pulp.
    • White pulp: rich in lymphocytes.
    • Red pulp: involved in blood filtration and red cell removal.
    • Composed of sinuses, splenic cords, and Stave cells.
    • Stave cells: specialized endothelial cells lining the splenic sinusoids.
    • Regulate immune responses and filter blood.

    Thymus

    • Primary lymphoid organ responsible for T cell maturation.
    • located in the mediastinum, behind the sternum.
    • Two lobes, each divided into lobules.
    • Contains a cortex and medulla.
    • Cortex: site of immature T cell development.
    • Medulla: home to mature T cells.

    MALT (Mucosa-Associated Lymphoid Tissue)

    • Lymphoid tissues dispersed throughout the mucosal linings of the body.
    • Protects against pathogens entering through mucosal surfaces.
    • Includes tonsils, Peyer's Patches, and appendix.

    Peyer's Patches

    • Lymphoid follicles found in the small intestine.
    • Contribute to immune surveillance in the digestive tract.

    Liver

    • Involved in immune function, but not considered a primary lymphoid organ.
    • Has Kupffer cells (macrophages) that engulf pathogens.
    • Produces acute phase proteins and other immune regulators.

    Adaptive Immune Response

    • Specific to a pathogen/invader
    • Slow to act at first, but becomes faster with subsequent exposure to the same pathogen due to memory cells
    • Mediated by B and T lymphocytes and antigen-presenting cells (APCs)
    • Two main types: Humoral and Cell-mediated

    Humoral Immune Response

    • Mediated by B lymphocytes
    • B lymphocytes differentiate into plasma cells that secrete antibodies
    • Antibodies are produced by plasma cells (IgA, IgE, IgM, IgG, IgD)

    Cell-Mediated Immune Response

    • T lymphocytes with the help of Major Histocompatibility Complex (MHC) eliminate the antigen
    • T lymphocytes eliminate the antigen

    Thymus

    • Enclosed by a capsule (C)
    • Trabecula or septa (S) divides the thymus into incompletely separated lobules
    • Each lobule has two distinct regions:
      • Cortex - outer, deeply staining (occupied by lymphoblasts)
      • Medulla - lightly staining
    • The cortex is densely packed with darkly staining T lymphoblasts that do not form lymphoid nodules
    • Many T lymphoblast cells in the cortex mature and enter the medulla after maturation
    • In adulthood, the thymus decreases in lymphoid tissue mass and cellularity, accompanied by an increase in adipose tissue

    Blood-Thymus Barrier

    • Prevents unregulated exposure of developing T lymphoblasts from antigens in the blood
    • If exposed, they might develop an autoimmune response
    • If a T lymphocyte binds to a MHC molecule, it survives and completes maturation to become CD4 (T helper cell) or CD8 (T cytotoxic cell)
    • If it does not bind, it undergoes apoptosis

    Medulla

    • Light staining with T lymphocytes
    • Contains fewer lymphocytes than the cortex
    • Hassall Corpuscles:
      • Made of large concentric aggregates of thymic epithelial cells (TEC)
      • Variable in size
      • Can be used as a landmark during lab exams (this would indicate that this is the Thymus)
      • Most characteristic feature of the medulla
      • Secrete cytokines
      • Regulate dendritic cell activity in the thymus
      • Promote development of regulatory T cells for peripheral tolerance

    T Cell Selection in the Thymus

    • First stage: Positive selection process that occurs in the cortex
    • The survival of the T lymphoblasts depends on its ability to bind to MHC molecules

    Spleen - White Pulp

    • Composed of:
      • Splenic/lymphoid nodule: contains cluster of B-lymphocytes with a germinal center
      • Central arteriole of the trabecular artery:
        • The term “Central Arteriole” is a misnomer because it is not really centrally-located but is found at the periphery of the lymphoid nodule
        • Pushed aside because of the proliferating lymphocytes
        • The central arteriole eventually leaves the white pulp and enters the red pulp, losing its sheath of lymphocytes and branching as several straight penicillar arterioles
        • This is the distinct characteristic of of a white pulp
      • The splenic artery branches to trabecular arteries that give off small branches called central arterioles that become enclosed with PALS within the white pulp, it branches into several penicillar arterioles which then lead to capillaries surrounded by macrophages.
      • From the capillaries, blood flows into either a closed circulation or to an open circulation

    Important Histological Comparisons of Major Lymphoid Organs

    • Thymus: Cortex and Medulla
    • MALT: Absent
    • Lymph Nodes: Present (cortex only)
    • Spleen: Cortex/Medulla (white pulp)
    • Thymus: Hassall Corpuscles
    • MALT: Crypts
    • Lymph Nodes: Paracortex, medullary cords & sinuses
    • Spleen: White pulp w/ central artery, red pulp with sinusoids

    DiGeorge Syndrome

    • Caused by the failure of the third and fourth pharyngeal pouches to develop, leading to thymic hypoplasia or aplasia.
    • Patients with DiGeorge syndrome have severely impaired cell-mediated immunity due to defective T lymphocyte production.

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