Immune & Lymphoid

• Immune system


  Provides defense or immunity against infectious agents ranging from viruses to multicellular parasites
• Immune system

  • Histologically, this system consists of a large, diverse population of leukocytes located within every tissue of the body and lymphoid organs interconnected only by the blood and lymphatic circulation
• Fundamental lines of defense

  • Innate immunity
  • Adaptive immunity
• Innate immunity


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


  • Includes 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 components

  • Hydrochloric acid (HCL)
  • Defensins, short cationic polypeptides produced by neutrophils and various epithelial cells that kill bacteria by disrupting the cell walls
  • Lysozyme, an enzyme made by neutrophils and cells of epithelial barriers, which hydrolyzes bacterial cell wall components, killing those cells
  • Complement, a system of proteins in blood plasma, mucus, and macrophages that react with bacterial surface components to aid removal of bacteria
  • Interferons, paracrine factors from leukocytes and virus-infected cells that signal NK cells to kill such cells and adjacent cells to resist viral infection
• Adaptive immunity


  • Acquired gradually by exposure to microorganisms
  • More specific (innate is non-specific)
  • Develops more slowly and is based on antigen presentation to lymphocytes
  • 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
• 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)


  • 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 have the common task of presenting peptides to be recognized by T cells on the cell surface
• Humoral immunity


  Antibodies are primarily responsible for the response
• Cell-mediated immunity


  Mature T cells lymphocytes, macrophages, and the production of cytokines in response to an antigen are the main drivers
• Primary lymphoid organs

  • Bone marrow for B lymphocytes
  • Thymus for T lymphocytes
• Secondary lymphoid organs

  • 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
• Thymic cortex cell types

  • 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
• Thymic medulla components

  • 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]
• Mucosa-associated lymphoid tissue (MALT)


  • 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


  • 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
• Compartments of lymph nodes

  • 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, Medullary Sinuses, Hilum - where blood vessels and nerve(s) enter
• Spleen


  • Only lymphoid organ involved in filtration of blood, making it an important organ in defense against blood-borne antigens
  • It is also the main site of old erythrocyte destruction
  • Without a cortex/medulla structure instead two intermingled but functionally different regions: white pulp and red pulp
• Spleen regions

  • White pulp - 20% of the spleen, enclosed by periarteriolar lymphoid sheaths (PALS) of T cells
  • Red pulp - Filters blood, removes defective erythrocytes, recycles hemoglobin iron. Contains splenic cords (cords of Billroth) with macrophages, reticular cells and fibers, other leukocytes, and splenic sinusoids lined by unusual endothelial cells called stave cells that are elongated and aligned parallel to the blood flow