Hematology hi

Cards (30)

  • Lymphoid system
    Important in providing defense against infectious agents
  • Lymphoid system
    • Consists of leukocytes distributed all over the body; as in the blood, lymph, epithelial & connective tissue, and within lymphoid organs
  • Lymphocyte formation and maturation
    1. Stem cells divide and mature into T or B lymphocytes in primary (central) lymphoid organs: thymus and red bone marrow
    2. Activation and maturation of lymphocytes occur in secondary (peripheral) lymphoid organs: lymph nodes, spleen, and diffuse lymphoid tissue found in different organs (mucosa associated lymphoid tissue)
  • Primary (central) lymphoid organs
    • Thymus
    • Red bone marrow
  • Secondary (peripheral) lymphoid organs
    • Lymph nodes
    • Spleen
    • Diffuse lymphoid tissue found in different organs (mucosa associated lymphoid tissue)
  • Tissue grafts are classified into:
    1. autograft: when donor and host are the same individual, as in burn patient. Skin is moved from undamaged to damaged regions.
    2- isograft: in identical twins.
    Both these grafts will not be rejected.
    3- homograft (allograft): involve two related or unrelated individuals. Host T cells may recognize the graft as foreign, leading to immune rejection.
    Immunosuppressive drugs such as cyclosporines inhibits the activation of cytotoxic T cells, to prevent rejection.
  • Thymus
    Is a bilobed structure in the mediastinum. It is a primary (central) lymphoid organ where T lymphocytes are produced. It plays a role in preventing autoimmunity.
    The thymus remains large and active until puberty, then begins to involute, and filled with adipose tissue
  • Thymus originates from endoderm germ layer ( third pharyngeal pouches).
    Medical application:
    Failure of development of thymus leads to Di George syndrome, which is characterized by thymic hypoplasia or aplasia. The individuals cant produce T lymphocytes properly, so develop severe depressed cell- mediated immunit
  • Lymphoid tissue
    Lymphoid tissue is reticular CT filled with large number of lymphocytes. It could be diffuse, or surrounded by capsule forming secondary lymphoid organ, where lymphocytes are supported by rich reticulin fibers. These fibers are produced by fibroblastic reticular cells. Lymphoid tissue stains dark blue in H&E stain, because lymphocytes have prominent basophilic nuclei and little cytoplasm.
  • One of the primary causes of the immunodeficiency syndrome known as AIDS (acquired immune deficiency syndrome) involves the killing of helper T cells by the infecting retrovirus.
    This cripples patients' immune systems rendering them susceptible to opportunistic infections by microorganisms that usually do not cause disease in immunocompetent individuals.
  • Lymphatic nodule(follicle)
    Small collections of densely-packed lymphocytes, contained within meshwork of reticular tissue, that appear strongly stained by haematoxylin in histological section, due to their basophilic nuclei.
    ➢Primary nodule: appears homogenous.
    ➢Secondary nodule: inner region of the nodule shows a less stained area called germinal center, which contains large lymphoblasts, with pale staining nuclei and larger amount of cytoplasm.
  • The thymus is surrounded by a vascularized connective tissue capsule that penetrate the parenchyma, and divides it into incomplete lobules.
    Each lobule has a peripheral dark zone known as the cortex, due to presence of large number of lymphoblasts and lymphocytes,
    and a central lighter zone, the medulla
  • Thymus/ cortex
    is composed of extensive population of T lymphoblasts, which also known as thymocytes, and thymic epithelial cells(TECs).
    TECs: have both features of epithelial and reticular cells. They have large euchromatic nuclei.
  • Types of TECs
    1-Squamous TECs: form a layer, and joined by desmosomes and occluding junctions that lines capsule, septa, and surrounds blood vessels.
    Squamous TEC, together with endothelial cells that line blood capillaries, and pericytes form blood- thymus barrier.
    this barrier forms an isolated cortical compartment, preventing thymocytes from exposing to antigens.
  • Types of TECs:
    2- cortical squamous TECs: form a sheetlike structure called cortico- medullary barrier, between cortex and medulla of thymic lobule.
    3- stellate TECs: have cytoplasmic processes, contain keratin tonofilaments, and joined by desmosomes. They act as antigen- presenting cells, and secrete cytokines which are important for T lymphocytes development.
  • Thymus/ Medulla
    Lightly stained area, because it contains fewer and larger mature lymphocytes. It also contains TECs, which have the following types:
    1. second layer of cortico-medullary layer.
    2- cytoreeticulum supportive TECs: to support lymphocytes and dendritic cells.
    3- large TECs: arranged concentrically around keratinized core, forming Hassall corpuscle.
  • Thymus/ Medulla
    Hassall corpuscle: is unique to the medulla, about 100 μm in diameter.
    Its cells secrete cytokines that promote the development of regulatory T lymphocytes.
    There is no blood- thymus barrier in the medulla.
  • Mucosa- Associated Lymphoid Tissue(MALT):
    Secondary lymphoid structures, where lymphocytes are activated by antigen presentation.
    they present intensively at the inner mucosa of digestive, respiratory, and genito- urinary systems, because these areas are commonly invaded by pathogens. Lymphocytes within MALT are difussed in the CT. sometimes these cells present as aggregates, such as tonsils, Peyers patches in the ilium, and mucosa of appendix. The most common type of cells present in MALT is B lymphocytes.
  • Mucosa- Associated Lymphoid Tissue(MALT)
    Peyers patches: composed of dozens of nodules, without capsule.
    M cells: present within the simple columnar epithelial lining of Peyers patches. They have apical microfolds.
    In the basal part of M cells there are large intracellular pockets containing lymphocytes and dendritic cells. The apical surface of M cells traps antigens, and transfers them to the pockets, to get rid of them.
  • Lymph Node
    Bean shaped and has a C.T. capsule which sends trabeculae inside the node
  • Lymph Node
    • Acts as filter - prevents spread of microorganisms and tumor cells
    • Provide and facilitate plasma cell production
    • Has a convex side and concave depression (hilum) - where arteries and nerves enter and veins and efferent lymphatic vessels leave
  • Lymph Node Structure
    • Lymphocytes
    • Plasma cells
    • Macrophages
    • Dendritic cells
    • APCs (antigen presenting cells)
    • FDCs (follicular presenting cells)
  • Lymph Node Structure
    1. Outer cortex
    2. Intermediate paracortex
    3. Inner central medulla
  • Cortex
    • Subscapular sinus - beneath capsule, receives lymph from afferent lymphatic vessels and transports to cortical sinus. Lined by thin, discontinuous endothelium penetrated by reticulin fibers and dendritic cell processes
    • Lymphoid nodule - composed of helper T cells, B lymphocyte blasts and macrophages
  • Paracortex
    • Rich in T cells, no B cells, no lymphoid nodules
    • Has HEV (high endothelial venules) which are lined by cuboidal cells. Their apical surface contains glycoproteins and integrins facilitating movement of lymphocytes out of blood, into the paracortex. Where 90% of lymphocytes enter lymph node.
  • Medulla
    • Medullary Cords - branched, cord like masses of lymphoid tissue, contain T, B lymphocytes and plasma cells
    • Medullary Sinuses - dilated spaces lined with discontinuous endothelium (separates the medullary cords). Their lumen contains processes of reticular cells, act as filters. They converge at helium as efferent lymphatic vessel.
  • Blood & Lymphatic Circulation
    1. Afferent lymphatic vessel cross capsule of the node and pour lymph into the subsapsular sinus
    2. Lymph pass through cortical sinuses that run parallel to the trabeculae and into the interior of the node, where they reach medullary sinuses
    3. After filtration of lymph (macrophages and dendritic cells in sinuses) it will be collected by efferent lymphatic vessels at the hilum
  • Tonsils
    Large, irregular, lymphoid tissue masses (found in mucosa of post. oral cavity and nasopharynx)
  • Types of Tonsils
    • Palatine (2 in lat. walls of oral part of pharynx) - covered with SS epithelium with 10-120 invaginations (form crypts). Filled with lymphocytes and many secondary lymph nodules. Has a thick C.T. capsule which separates it from underlying structures thus preventing infection spread.
    • Pharyngeal (1 in post. Wall of nasopharynx) - covered with pseudostratified columnar epithelium. Consists of folds of mucosa with lymphoid tissue and nodules. No crypt and thin capsule
    • Lingual (small and multiple, at base of tongue) - covered with SS epithelium and has a crypt but no capsule
  • Clinical
    • Metastatic cancer cells - carried from primary tumor and enter lymph nodes where they growth as secondary tumors. Their presence is important in staging & prognosing the tumor
    • Lymphoma - neoplastic proliferation of lymphocytes in lymph nodes. Obliterates lymph node structure and make it enlarged (lymphadenopathy)
    • Tonsillitis - tonsil inflammation, common in children
    • Adenoid - chronic inflammation of pharyngeal lymphoid tissue & tonsils of children, leading to enlargement and hyperplasia of tonsils. Can obstruct Eustachian tube and cause middle ear infection.