Immune System and Lymphoid Organs

• Immune system provides immunity against infectious agents ranging from viruses to multicellular parasites.
• Histologically, this system consist of large, diverse population of leukocyte located within every tissue pf the body and lymphoid organs interconnected only by the blood and lymphatic circulation
• Innate Immunity

  • represents the first line of defense to an intruding pathogen
  • antigen independent (non-specific) defense mechanism that is used by the host immediately or within hours of encountering an antigen
• Innate Immunity

  • including physical barriers such as the skin and mucous membranes of the gastrointestinal, respiratory, and urogenital tracts that prevent infections or penetration of the host body
  • involves leukocytes (mainly granulocytes [basophil, neutrophil, eosinophil]), and proteins such as defensins, complement, lysozyme, and interferons; cytokines
• Innate Immunity

  • Hydrochloric Acid [HCL]
  • Defensis : short cationic polypeptides produced by neutrophils and various epithelial cells that kill bacteria by disrupting the cell walls.
• Innate Immunity

  • Lysozyme : an enzyme made by neutrophil and cells of epithelial barriers, which hydrolyzes bacterial cell wall components, killing those cells.
• Innate Immunity

  Complement : a system of proteins in blood plasma, mucus, and macrophages that reacts with bacterial surface components to aid removal of bacteria
• Innate Immunity

  Interferons : paracrine factors from leukocytes and virus infected cells that signal NK cells to kill such cells and adjacent cell to resist viral infection
• Adaptive Immunity

  • Acquired gradually by exposure to microorganisms,
  • More specific [innate: non-specific]
  • Develops more slowly and is based on antigen presentation to lymphocytes
• Adaptive Immunity

  • Responses are aimed at specific microbial invaders and involve production of memory lymphocytes so that a similar response can be mounted very rapidly if that invader ever appears again
  • Response involves B and T lymphocytes
• ANTIGENS

  Usually proteins; that are recognized by lymphocytes to elicit a specific immune response against them.
• ANTIBODIES

  immunoglobulins produced by plasma cells after a progenitor B cell is activated by a specific antigen
• Major Histocompatibility Complexes (MHC)

  • Are cell surface glycoproteins whose primary function is to present peptide fragments for recognition by the appropriate T cells (lymphocytes)
• MHC Class I Molecules - found on surfaces of all nucleated cells bear fragments of their constituent proteins
• MHC Class II Molecules – Only antigen- presenting cells (APCs)
• Both MHC Class I and II molecules have the common task of presenting peptides to be recognized by T cells on the cell surface
• HUMORAL IMMUNITY

  • which antibodies are primarily responsible for the response
  • B cell lymphocytes, a type of immune cellthat makes antibodies after detecting a specific antigen, are principally responsible for this method
• CELL-MEDIATED IMMUNITY

  • Mature T cells lymphocytes, macrophages, and the production of cytokines in response to an antigen are the main drivers of cell-mediated immunity.
• Primary Lymphoid Organs [where the lymphocytes mature and are functional]

  Bone marrow for B Lymphocytes

  • Thymus for T lymphocytes
• Secondary Lymphoid Organs [this is where the lymphocytes circulate after maturation]

  - Where B and T cells are often activated, proliferate, and begin to function

  - Contains a meshwork of reticulin produced by fibroblastic reticular cells,

  • Lymph nodes
  • MALT
  • Spleen
• THYMUS

  • Bilobed organ in the mediastinum that is most active and prominent before puberty and undergoes involution with less activity in the adult.
  • found in the midline of the thoracic cavity, that is surrounded by the left and right pleural sacs
  • Primary organ where T Cells are produced
  • Originates from endoderm
• THYMIC CORTEX

  • darkly basophilic
  • contains an extensive population of T lymphoblasts (or thymocytes) located among numerous macrophages and associated with the unique thymic epithelial cells (TECs) that have certain features of both epithelial and reticular cells.
• SQUAMOUS CELLS

  • Blood-thymus barrier that is responsible in preventing unregulated exposure of thymocyte to the antigens
• STELLATE EPITHELIAL CELLS

  • Cytoreticulum, secrete numerous cytokines for T-cell development
• SQUAMOUS CORTICAL CELLS

  • Corticomedullary barrier
• THYMIC MEDULLA

  • contains fewer and larger, more mature lymphocytes.
• CYTORETICULUM

  • supports T lymphocytes, dendritic cells and macrophages; expresses many specialized proteins specific to cells of other organs
• SECONDARY LAYER

  • serves as boundary between cortex and medulla
• HASSAL CORPUSCLES

  • Aggregates of TEC [Thymic Epithelial Cells]
• MALT is found in the mucosa of most tracts but is concentrated in the palatine, lingual and pharyngeal tonsils, Peyer patches, and the appendix.
• Collectively the MALT is one of the largest lymphoid organs, containing up to 70% of all the body’s immune cells. Most of the lymphocytes here are B cells; among T cells, CD4+ helper T cells predominate
• Lymph nodes are bean-shaped, encapsulated structures, generally only 10 mm by 2.5 cm in size, distributed throughout the body along the lymphatic vessels

  • Filters lymph
  • Site for B-cell activation and differentiation
• OUTER CORTEX

  • point of entry of lymphocytes to the entire Lymph Node
  • Where B cells encounter antibodies
• PARACORTEX

  • High Endothelial Venules (HEVs) portal of entry of lymphocytes to paracortex
• INNER MEDULLA

  • Medullary Cords
  • Medullar Sinuses
  • Hilum – where blood vessels and nerve(s) enter
• Spleen is the only lymphoid organ involved in filtration of blood,

  making it an important organ in defense against blood-borne antigens.
• Spleen is also the main site of old erythrocyte destruction.
• Spleen without a cortex/medulla structure instead two intermingled but functionally different regions: white pulp and red pulp.
• WHITE PULPS

  • 20% of the spleen
  • Enclosed by periarteriolar lymphoid sheaths (PALS) of T cells.
• RED PULP

  • Filters blood,
  • Removes defective erythrocytes,
  • Recycles hemoglobin iron