Lymphatic System

• Lymphatic System

  • Group of cells, tissues, and organs that monitor body surfaces and internal fluid and reactant to potentially harmful substances
  • Also known as lymphoid system
  • Consists of a huge network of lymph nodes, lymphatic vessels, lymphoid or lymphatic organs, and lymphoid tissue cells 
• • Innate immunity - nonspecific, involves a wide variety of effector mechanisms, and older than adaptive immunity
• Innate immunity

  • Including physical barriers such as the skin, and mucus membranes of the gastrointestinal, respiratory, and urogenital tracts that prevent infections
• Innate immunity 

  Neutrophils - bacteria, fungi, and parasites that manage to penetrate these barriers (skin or mucus membrane) are removed by this leukocyte
• Innate immunity

  • Natural killer (NK) cells - destroy various host cells, including those infected with virus or bacteria or certain potentially tumorigenic cells 
• Adaptive immunity - aims at specific microbial invaders; mediated by lymphocytes and antigen-presenting cells (APCs), and produces memory cells that permit a similar, very rapid response if the specific microbe appears again 
• Lymphocytes - formed initially in primary lymphoid organs (thymus and bone marrow)
• Lymphocytes activation and proliferation occur in secondary lymphoid organs (lymph nodes, spleen, and diffuse lymphoid tissue)
• Mucosa-associated lymphoid tissue (MALT) - immune cells located diffusely in the digestive, respiratory, or urogenital mucosae 
• • INNATE IMMUNITY Hydrochloric acid (HCl) and organic acids regions lower the pH locally to either kill entering microorganisms directly or inhibit their growth
• Defensins short cationic polypeptides produced by neutrophils and various epithelial cells that kill bacteria by disrupting the cell walls 
• Lysozyme enzyme made by neutrophils and cells of epithelial barriers, which hydrolyzes bacterial cell wall components, killing cells
• • Complement 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
  • Specific
  • Slower to respond 
  • More recent development 
  • Involves B and T lymphocytes

  Aimed at specific microbial invaders and involve production of memory lymphocytes so that similar response can be mounted very rapidly if the invader appears again
• APCs - derived from monocytes
• Cytokines - cell communication center in the lymphoid organs occurring during inflammation at sites of infection or tissue injury to coordinate defensive measures 

  • Involves both innate and adaptive immunity 
  • Diverse group of polypeptides and glycoproteins
• Chemotaxis - directed cell movements during diapedesis
• Interleukins - stimulation or suppression of lymphocyte activities in adaptive immunity
• • Antigen a molecule that is recognized by cells of the adaptive immune system eliciting a response from the cells
• Epitopes - small molecular domains of the antigen known as antigenic determinants
• Antibody - glycoprotein of the immunoglobulin family that interacts specifically with an antigenic determinant
• Antibody

  Secreted by plasma cells
• Immunoglobulins: two identical light chains and two identical heavy chains bound by disulfide bonds
• Five major classes of antibodies:

  1. IgG
  2. IgA
  3. IgM
  4. IgE
  5. IgD
• IgG - most abundant class representing 75% - 85% of the immunoglobulin in blood 

  • Production increases during immune responses after infections 
  • Unlike others, it is highly soluble, stable, and crosses placental barrier into the fetal circulation 
  • Passive immunity 
• IgA - present in almost all exocrine secretions as a dimeric form in which the heavy chains of two monomers united by a polypeptide called the J chain

  • Produced by plasma cells 
  • Bound by secretory component 
• IgM - 5%-10% of blood immunoglobulin; exits in a pentameric form united by J chain 
• IgM - 5%-10% of blood immunoglobulin; exits in a pentameric form united by J chain 

  • Produced in an initial response to an antigen 
  • The most effective antigen class in activating complement system 
• IgE - monomer; least abundant in the circulation 

  • Triggers the release of histamine, heparin, and leukotrienes 
  • Allergic reaction
• IgD - least abundant immunoglobulin in plasma; least understood class of antibody 

  • Bound to the surface of B lymphocytes where they act as antigen receptors in triggering B-cell activation
• Opsonization - ability of receptors on macrophages, neutrophils, and eosinophils to recognize and bind the Fc portions of antibodies attached to surface antigens 

  • Phagocytosis
• NK cells activation - antibodies bound to antigens on virus-infected cells of the body are recognized by the primitive lymphocytes called NK cells

  • NK cells - activated to kill the infected cell by releasing perforin and granzymes
• Major histocompatibility complex (MHC) - abundant antigen-presenting proteins are part of this.
• • MHC class I and class II
  • These proteins were first recognized by their roles in the immune rejection of grafted tissue or organs
• Protein of MHC I and II: human leukocyte antigens (HLAs)
• T lymphocytes - specialized to recognize both classes of MHC proteins and antigens
• MHC molecules - made in the rough ER and Golgi apparatus
• Gene mutation also have MHC class I proteins displaying peptides that T cells do not recognize as self, helping lead to the elimination
• MHC class II - synthesized and transported to the cell surface but only in cells of mononuclear phagocyte system and certain other cells under some conditions