Lymphatic system

• Lymphatic system


  Consists of lymph, lymphatic vessels, several structures including organs containing lymphatic tissue, such as spleen, thymus, lymph nodes, lymph nodules, and red bone marrow
• Lymphatic tissue


  A specialized form of reticular connective tissue that contains large numbers of lymphocytes
• Lymphatic vessels


  Lymphatic capillaries in tissue spaces between cells merge to form larger lymphatic vessels, which ultimately drain into the thoracic duct or right lymphatic duct
• Thoracic duct and right lymphatic duct


  Thoracic duct empties its lymph into the left internal jugular and left subclavian veins, and the right lymphatic duct empties its lymph into the right internal jugular and right subclavian veins
• The thoracic duct drains a much larger portion of the body than does the right lymphatic duct
• Lymph


  Fluid that bathes the cells of body tissues, chemically similar to blood plasma
• Primary functions of the lymphatic system

  • Drains excess interstitial fluid
  • Transports dietary lipids
  • Carries out immune responses
• Lymphatic vessels and lymph nodes


  Located at intervals along lymphatic vessels are lymph nodes, masses of B cells and T cells surrounded by a capsule. Lymph flows through lymph nodes.
• Flow of lymph


  1. Respiratory pump - during inspiration, flow of lymph increases as the thoracic cavity's pressure becomes more negative

  2. Skeletal muscle pump - when muscles contract, the valves are forced open to increase the lymph flow
• Primary lymphatic organs


  Sites where stem cells divide and develop into mature B cells and T cells. They include the red bone marrow and the thymus.
• Red bone marrow


  Fills the head of the femur, and a spot of yellow bone marrow is visible in the center
• Thymus


  A two-lobed organ located posterior to the sternum, medial to the lungs, and superior to the heart. It contains large numbers of T cells and macrophages.
• Thymic lobules


  • Separated by connective tissue septae. Each lobule is made up of a peripheral cortex and an inner medulla.
• Inner medulla of thymus

  Thymic epithelial cells congregated into Hassall's corpuscles. Thymocytes - the regulatory T-cells differentiate into lymphocytes.
• Function of thymus


  Maturation and education of T lymphocytes via positive and negative selection
• Other cells in thymus

  Myeloid cells include monocytes, macrophages, and dendritic cells. Fibroblasts facilitate the production of collagenous material.
• Secondary lymphatic organs and tissues

  Sites where most immune responses occur. They include lymph nodes, the spleen, and lymphatic nodules.
• Lymph nodes


  Located along lymphatic vessels and are heavily concentrated near the mammary glands and in the axillae and groin. They contain B cells that develop into plasma cells, T cells, dendritic cells, and macrophages.
• Lymphatic nodules


  Egg-shaped masses of lymphatic tissue that are not surrounded by a capsule. They are plentiful in the connective tissue of mucous membranes lining the gastrointestinal, urinary, and reproductive tracts and the respiratory airways.
• Spleen


  The largest single mass of lymphatic tissue in the body. It lies between the stomach and diaphragm and is covered by a capsule of dense connective tissue.
• Histology of the spleen

  Contains two types of tissue: white pulp (lymphatic tissue where B and T cells carry out immune responses) and red pulp (blood-filled sinuses where worn-out blood cells and platelets are removed).
• Tonsils


  A pair of soft tissue masses located at the rear of the throat (pharynx). Each tonsil is composed of tissue similar to lymph nodes, covered by pink mucosa. Running through the mucosa of each tonsil are pits, called crypts.
• Function of tonsils


  Immune responses against inhaled or ingested foreign substances.
• Immunity


  The ability to use our body's defenses to ward off damage or disease
• Innate (nonspecific) immunity


  Defenses that are present at birth
• Adaptive (specific) immunity


  Defenses that involve specific recognition of a microbe
• Adaptive immunity


  Involves lymphocytes called T lymphocytes (T cells) and B lymphocytes (B cells)
• Components of innate immunity

  • External physical and chemical barriers provided by the skin and mucous membranes
  • Antimicrobial substances
  • Natural killer cells
  • Phagocytes
  • Inflammation
  • Fever
• Skin and mucous membranes

  First line of defense against pathogens, providing both physical and chemical barriers
• Antimicrobial substances in body fluids

  • Interferons
  • Complement system
  • Iron-binding proteins
  • Antimicrobial proteins (AMPs)
• Phagocytes and natural killer cells


  Second line of defense when microbes penetrate the skin and mucous membranes or bypass the antimicrobial substances in blood
• Inflammation


  A nonspecific defensive response of the body to tissue damage
• Fever


  An elevated body temperature that intensifies the effects of interferons, inhibits the growth of some microbes, and speeds up body reactions that aid repair
• Symptoms of inflammation


  • Pain at the injury site
  • Redness
  • Heat
• First line of defense: Skin and mucous membranes

  • Physical factors: Epidermis of skin, Mucous membranes, Mucus, Hairs, Cilia, Lacrimal apparatus, Saliva, Urine, Defecation and vomiting
  • Chemical factors: Sebum, Lysozyme, Gastric juice, Vaginal secretions
• Second line of defense: Internal defenses

  • Antimicrobial substances: Interferons (IFNs), Complement system, Iron-binding proteins, Antimicrobial proteins (AMPs)
  • Natural killer (NK) cells
  • Phagocytes
  • Inflammation
  • Fever
• Antigen


  Any substance (foods, microbes, drugs, pollen, etc.) that the immune system recognizes as foreign (non-self)
• Antibody-mediated immunity


  B cells transform into plasma cells, then into antibodies
• Cell-mediated immunity


  Cytotoxic T cells directly attack invading antigens
• Antibodies


  Plasma proteins belonging to a group called immunoglobulins, grouped in five different classes