Immunology Basics: Lymphatic and Thymus Cells

Lymphatic System

• The lymphatic system is a network of vessels, structures, and organs containing lymphatic tissue.
• It also involves red bone marrow.
• Its primary functions are:
  • Draining excess interstitial fluid.
  • Transporting dietary lipids.
  • Carrying out immune responses.

Immunity or Resistance

• Immunity is the body's ability to fight off damage or disease through its defenses.
• There are two types of immunity:
  • Innate (non-specific) immunity is present at birth and does not recognize invaders or have memory of them.
  • Adaptive (specific) immunity specifically recognizes invaders and has memory of them.
  • Innate immunity has 1st and 2nd lines of defense.

Lymphatic Vessels and Lymph Circulation

• Lymphatic vessels start as capillaries, closed at one end.
• They merge into larger vessels that resemble veins, having thinner walls and more valves.
• Lymphatic vessels pass through lymph nodes and encapsulated vessels with B and T cells.

Lymphatic Capillaries

• Lymphatic capillaries have larger diameters than blood capillaries.
• They're exclusively one-way, allowing interstitial fluid to enter but not exit.
• Anchoring filaments expand openings when fluid accumulates.

Lymph Trunks and Ducts

• Lymphatic vessels merge to form trunks.
• Principal trunks include lumbar, intestinal, bronchomediastinal, subclavian, and jugular.
• Lymph trunks drain into two main channels (thoracic and right lymphatic ducts) that empty into venous blood.

Lymphoid Cells

• Lymphocytes are the primary cells involved in the immune response.
• The major varieties include T cells and B cells.

T cells and B cells

• T cells and B cells protect the body against antigens.
• Antigens are any substances the body perceives as foreign, including bacteria, toxins, viruses, and cancer cells.
• T cells manage the immune response and attack foreign cells.
• B cells produce plasma cells that secrete antibodies, immobilizing antigens.

Other Lymphoid Cells

• Macrophages phagocytize foreign substances and help activate T cells.
• Dendritic cells are spiny-like cells similar to macrophages that capture antigens and bring them back to lymph nodes.
• Reticular cells are fibroblast-like cells that create a stroma supporting lymphoid cells in organs.

Lymphatic Tissues and Organs

• Two groups based on function:
  • Primary Lymphatic Organs
    • Sites for stem cell division and development of immunocompetence (red bone marrow and thymus).
  • Secondary Lymphatic Organs
    • Sites for most immune responses (lymph nodes, spleen, lymphatic nodules).

Lymphoid Organs - Specific Examples

• Thymus:
  • Contains an outer cortex and an inner medulla.
  • Cortex densely packs lymphocytes and macrophages; medulla has fewer lymphocytes and thymic corpuscles.
  • Maintains T-cell maturation.
• Lymph Nodes:
  • Located along lymphatic vessels, scattered throughout the body.
  • Contains stroma (capsule, fibers, and trabeculae) and parenchyma (cortex and medulla) involved in immune responses by filtering lymph and filtering antigens
  • Lymph enters through afferent vessels and exits through efferent vessels. Two distinct regions exist cortex and medulla. Cortex contains follicles with germinal centers (dividing B cells); inner cortex houses T cells, and medulla houses B cells, plasma cells, and macrophages.
• Spleen:
  • Largest mass of lymphatic tissue.
  • Stroma consists of capsule, trabeculae, reticular fibers, and fibroblasts.
• Parenchyma:
• Contains white pulp (lymphocytes, macrophages) and red pulp (blood-filled sinuses and splenic cords containing red blood cells, macrophages, lymphocytes, plasma cells, and granulocytes)
• Lymph nodes and spleen filter lymph and blood, respectively.
• Red pulp removes worn-out blood cells.
• Spleen also serves as a blood reservoir and site of fetal blood cell production.

Lymphatic Nodules

• Scattered throughout the lamina propria of mucous membranes.
• Mucosa-associated lymphatic tissue (MALT) is in places like respiratory tract.
• Some nodules, like tonsils, Peyer's patches, and the appendix, are larger.

Formation and Flow of Lymph

• More fluid filters out of blood capillaries than is reabsorbed.
• Excess fluid becomes lymph, about 3 liters per day, draining into lymphatic vessels.
• Lymphatic vessels return lost plasma proteins to the bloodstream.
• Lymphatic vessels have valves, aiding lymphatic return.
• Skeletal muscle and respiratory pumps assist venous return.

Relationship of the Lymphatic System to the Cardiovascular System

• Lymph vessels and lymph nodes connect to the cardiovascular system.
• Lymph travels to venous circulation.

Overview of Innate Immunity

• A first line of defense of the body against invaders consisting of physical and chemical barriers.
• A second line of defense of the body against invaders consisting of cellular and molecular mechanisms which act internally. Ex. Inflammatory response, Natural killer cells and phagocytes.
• Inflammation as a second line of defense in the body against cell damage

Adaptive Immunity

• The adaptive defense mechanism acts against specific pathogens and produces cellular and molecular mechanisms.

Antigen Processing Pathways

• B cells and T cells respond to antigens in distinct ways and utilize different antigen processing pathways based on whether the antigen is outside or inside the body's cells.