LYMPHOID

• Lymphoid System

  A complex network of ducts, nodes and other organs that are located throughout the body
• Lymphoid System


  • First line of defense
  • Inflammatory response
  • Immune response
• Physical Barriers

  • Skin
  • Mucus Membranes (mucosae)
• Skin cannot normally be penetrated by bacteria and viruses
• Secretions from sebaceous and sweat glands keep the skin in a pH range of 3 to 5 (acidic)
• Microbial colonization is also inhibited by saliva, tears, and mucus secretions that continually bathe exposed epithelium
• All of these secretions contain antimicrobial proteins, e.g. lysozyme, an enzyme that digests the cell walls of many bacteria
• Mucus Membranes (mucosae) line digestive, respiratory, and genitourinary tracts and prevent entry of harmful microbes
• In the trachea, ciliated epithelial cells sweep out mucus and trapped microbes
• Inflammatory Response


  Tissue damages leads to a localized inflammatory response (nonimmune response; inflammation)
• Inflammatory Response


  • Increased dilation
  • Increased permeability
  • Leads to increased redness, heat, and swelling
• Effector Cells


  • Neutrophils
  • Macrophages
  • Eosinophils
  • Basophils
  • Mast cells
  • NK cells
  • T cells
• Neutrophils are the first phagocytes to arrive, followed by macrophages
• The dead phagocytic cells and fluids leaked from capillaries is called pus
• Complement System


  A collection of more than 20 plasma proteins that produced by the liver, which enhances phagocytosis and involved in both inflammatory and immune response
• Activation of the Complement System

  • Production of chemotaxins
  • Marking off bacteria with proteins (opsonins)
  • Facilitate phagocytosis
  • Release of cytokines
  • Release of histamine
• Histamine


  A chemical mediator secreted by mast cells and basophils that triggers increased dilation and permeability of nearby capillaries
• Damaged tissues also release prostaglandins and other substances that promote blood flow to the injured site
• Systemic Response


  A widespread non-specific response that may occur if damage or infection is severe, including an increase in the number of leukocytes in the blood
• Fever


  A systemic response characterized by increased body temperature, which may be triggered by toxins or by pyrogens released from leukocytes
• Immune Response


  A more powerful body defense system than the inflammatory response, which is antigen-specific and must be developed
• Antigen


  Any substance that the immune system perceives as foreign to the body and which, consequently, induces an immune response
• Lymphocytes


  The principal effector cells of the immune response
• Types of Immune Responses

  • Humoral Immunity (Antibody-Mediated Immunity)
  • Cell-Mediated Immunity (CMI)
• Humoral Immunity


  Immunity mediated by antibodies, which are substances synthesized by plasma cells
• Cell-Mediated Immunity


  Immunity not mediated by antibodies, which plays a major role in conferring immunity to microorganisms located intercellularly
• Effector Cells


  • Cytotoxic T Cells
  • Macrophages
• Cytotoxic T Cells


  Target virus-infected cells, cells with intracellular bacteria, and cancer cells, and can induce apoptosis of the target cell
• Apoptosis


  A form of cell death in which a programmed sequence of events leads to the elimination of cells without releasing harmful substances into the surrounding area
• Macrophages normally digest many pathogens with their lysozomal enzymes without help from Th cells
• Primary Immune Response


  A minor reaction with a long induction phase (several days)
• Secondary Immune Response


  The response elicited by re-exposure to an antigen that has previously triggered a primary immune response
• The immune response is vital to survival, but it sometimes go awry, causing allergic reactions that are occasionally fatal, as in cases of anaphylactic shock following bee stings
• Components of Lymphoid System

  • Central/Primary Lymphoid Organs
  • Peripheral/Secondary Lymphoid Organs
• Central/Primary Lymphoid Organs


  The thymus and the bone marrow, involved in the production and early selection of lymphocytes
• Peripheral/Secondary Lymphoid Organs


  Provide the environment for foreign or altered native molecules (antigens) to interact with lymphocytes, e.g. lymph nodes, lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin
• Lymphoid Tissue


  Consists of a connective tissue framework (stroma) and the functional cellular elements that it supports (parenchyma), existing in the body in the form of diffuse lymphoid tissue or lymphoid nodule
• Diffuse Lymphoid Tissue


  Lymphocytes are evenly dispersed, forming part of lymphoid organs and connective tissue, especially prominent in the lamina propria and submucosa of gastrointestinal, respiratory, and genitourinary tracts
• Lymphoid Nodule


  Lymphoid nodules begin to appear only at birth, usually interspersed in areas of diffuse lymphoid tissue, and can occur singly (solitary nodules) or in aggregates (Peyer's patches)
• Primary Lymphoid Nodules


  Do not have germinal centers, containing small idle or resting lymphocytes evenly distributed throughout the nodule