Immune System and Lymphoid Organs

• Immune System:

  • Provides defense or immunity against infectious agent
  • Diverse population of leukocyte within lymphoid organs
• Two fundamental lines of defense:

  1. Innate Immunity
  2. Adaptive Immunity
• Innate Immunity:

  • first line of defense
  • antigen independent defense mechanism
  • non specific
  • used by the hos immediately or within hours
  • Physical barriers such as skin, mucous membrane of gastrointestinal, respiratory and urogenital tracts
  • Involves basophil, neutrophil and eosinophil and proteins such as defensins, complement, lysozyme and interferons or cytokines.
• It is short cationic polypeptides produced by neutrophils and various epithelial cells that kill bacteria by disrupting the cell wall?

  defensins
• An enzyme made by neutrophils and cells of epithelial barriers, which hydrolyzes bacterial cell wall components, killing those bacteria

  Lysozyme
• a system of proteins in blood plasma, mucus and macrophages that react with bacterial surface components to aid removal of bacteria

  complement
• paracrine factors from leukocytes and virus-infected cells that signal NK cells to kill such cells and adjacent cells to resist viral infection

  interferons
• Adaptive Immunity:

  • Acquired gradually by exposure
  • More specific
  • Develops more slowly
  • Involve production of memory lymphocytes
  • Response B and T lymphocytes
• Usually proteins; that are recognized by lymphocytes to elicit a specific immune response against them.

  antigens
• immunoglobulins produced by plasma cells after progenitor B cell is activated by a specific antigen

  antibodies
• are cell surface glycoproteins whose primary function is to present peptide fragments for recognition by the appropriate T cells.

  MHC
• found on surfaces of all nucleated cells bear fragments of their constituent protein


  MHC class I molecules
• only antigen presenting cells

  MHC class II molecules
• Primary responsible for the response

  Humoral Immunity
• B cell lymphocytes, a type of immune cell that makes antibodies after detecting a specific antigen are principally responsible for this method

  Humoral immunity
• Mature T lymphocytes, macrophages and the production of cytokines in response to an antigen are the main drivers.

  Cell-mediated immunity
• What are the primary Lymphoid organs?

  Bone marrow and Thymus
• What are the Secondary Lymphoid organs?

  MALT, spleen, lymph nodes
• B lymphocytes are produced by?

  bone marrow
• T lymphocytes are produced by?

  Thymus
• Thymus:

  • Bilobed organ in the mediastinum that is prominent before puberty
  • Found in midline of thoracic cavity
  • T cells are produced
  • Originate from endoderm
• Enumerate Thymic Cortex:

  • Squamous cells
  • Stellate epithelial cells
  • Squamous cortical cells
• Thymic Cortex:

  • Darkly basophilic
  • contains an extensive population of T lymphoblasts or thymocytes and associated with the unique thymic epithelial cells
• It is blood-thymus barrier that is responsible in preventing unregulated exposure of thymocytes to the antigens.

  squamous cells
• Cytoreticulum, secrete numerous cytokines for T-cell development

  Stellate Epithelial Cells
• Corticomedullary barrier


  Squamous cortical cells
• Contains fewer and larger, more mature lymphocytes

  Thymic medulla
• Enumerate Thymic Medulla:

  • Cytoreticulum
  • Secondary layer
  • Hassal Corpuscle
• Supports T lymphocyte, dendritic cells and macrophages; expresses many specialized proteins specific to cell of other organs

  Cytoreticulum
• Serves as boundary between cortex and medulla

  Secondary layer
• Aggregates TEC


  Hassal corpuscles
• Where does MALT found?

  palatine, lingual, pharyngeal tonsils, peyer patches, appendix
• It is one of the largest lymphoid organ. Most of the lymphocytes heare are B cells; among T cells, CD4+ helper T cells predominate

  MALT
• It is bean-shaped encapsulated structures


  Lymph nodes
• It filters lymph

  Lymph nodes
• Site for B-cell activation and differentiation

  Lymph nodes
• Compartments of Lymph nodes:

  • Outer cortex
  • Paracortex
  • Inner medulla
• • It is point of entry of lymphocytes to the entire lymph node
  • Where B cells encounter antibodies


  Outer cortex
• High Endothelial Venules portal of entry of lymphocytes to paracortex

  Paracortex
• • Medullary cords
  • Medullar sinuses
  • Hilum

  Inner Medulla