9-13 Immune System

Lymphatic (Immune) System. Lymph vessels – Microscopic, Ultramicroscopic and Functional Characteristics of the Wall.

• A vascular system that returns interstitial fluid to the blood.

• Participates in immune surveillance, fat absorption, and fluid balance.

Microscopic Characteristics of the Wall:

• Resemble thin-walled veins, but with more valves and thinner walls.

• Three basic layers (less distinct than in blood vessels):

  1. Tunica intima: endothelium + thin connective tissue.

  2. Tunica media: thin layer of smooth muscle.

  3. Tunica adventitia: connective tissue with collagen and occasional elastic fibers.

• Larger lymph vessels (e.g. thoracic duct) have vasa vasorum and smooth muscle.

Ultramicroscopic Characteristics:

• Endothelial cells:

  • Overlapping flap-like junctions act as one-way valves—open with high interstitial pressure.

  • Anchored to surrounding tissue by anchoring filaments to prevent collapse.

• No basement membrane or a very discontinuous one, allowing easy fluid entry.

• Smooth muscle cells in larger vessels show rhythmic contractions (lymphangion function).

Functional Characteristics:

• Collect and transport lymph (interstitial fluid, proteins, immune cells) back to the venous system.

• Valves ensure unidirectional flow toward the heart.

• Lymphangions (segments between valves) contract rhythmically to propel lymph.

• Important in immune function, absorbing lipids from intestines (via lacteals), and maintaining fluid balance.

Lymphatic (Immune) System. Lymph

• Definition: Clear to slightly yellow fluid derived from interstitial fluid.

• Composition:

  • Water, proteins, electrolytes, waste products, lipids (chyle) from intestines.

  • Contains lymphocytes and antigen-presenting cells (e.g. dendritic cells, macrophages).

• Function:

  • Transports immune cells to lymph nodes for antigen screening.

  • Returns excess tissue fluid to the bloodstream.

  • Transports fats from the small intestine via lacteals into the thoracic duct.

Lymphatic (Immune) System. Lymph Organs. Classification.

1. Primary (Central) Lymphoid Organs:

• Bone marrow – site of B cell development and hematopoiesis.

• Thymus – site of T cell maturation.

2. Secondary (Peripheral) Lymphoid Organs:

• Lymph nodes – filter lymph and mount immune responses.

• Spleen – filters blood; immune response against blood-borne antigens.

• Mucosa-associated lymphoid tissue (MALT) – includes tonsils, Peyer's patches, appendix.

Lymphatic (Immune) System. General Microscopic, Ultramicroscopic and Functional Characteristics of the Organs.

1. Thymus (Primary Organ)

Microscopic Characteristics:

• Lobulated organ with cortex and medulla.

• Cortex: dense with immature T cells (thymocytes) and epithelial reticular cells.

• Medulla: fewer lymphocytes; contains Hassall’s corpuscles.

Ultramicroscopic Features:

• Epithelial reticular cells with desmosomes, tonofilaments, and antigen-presenting capacity.

• Blood–thymus barrier in cortex protects maturing T cells from exposure.

Functional Role:

• Site of T cell maturation and selection.

• Most active in childhood, involutes with age.

2. Lymph Nodes (Secondary Organ)

Microscopic Characteristics:

• Cortex: contains lymphoid follicles with germinal centers (B cells).

• Paracortex: rich in T cells and high endothelial venules (HEVs).

• Medulla: medullary cords (B cells, macrophages) and sinuses (lymph flow).

Ultramicroscopic Features:

• Reticular fibers form a supportive framework.

• Endothelial cells in HEVs allow lymphocyte entry from blood.

Functional Role:

• Filter lymph, trap antigens, initiate immune response.

• Site for B and T cell activation and proliferation.

3. Spleen (Secondary Organ)

Microscopic Characteristics:

• White pulp: lymphoid tissue around central arterioles (T and B cell zones).

• Red pulp: sinusoids and splenic cords; contains macrophages and RBCs.

Ultramicroscopic Features:

• Sinusoids with discontinuous endothelium (open circulation model).

• Macrophages and dendritic cells trap blood-borne antigens.

Functional Role:

• Filters blood, destroys old RBCs, stores platelets.

• Mounts immune responses to blood-borne pathogens.

4. MALT (Mucosa-Associated Lymphoid Tissue)

Microscopic Characteristics:

• Lymphoid follicles in mucosa/submucosa of organs (GI, respiratory, urogenital).

• No capsule; includes Peyer’s patches, tonsils, and appendix.

Ultramicroscopic Features:

• M cells (in Peyer's patches) transport antigens to immune cells.

• Contains plasma cells producing IgA.

Functional Role:

• Provides localized immune defense at mucosal surfaces.

• First line of defense against inhaled or ingested antigens.

Lymph nodes. Macroscopic, Microscopic, Ultramicroscopic and Functional Characteristics.

Macroscopic Characteristics:

• Small, bean-shaped organs (~1–25 mm).

• Located along lymphatic vessels; clustered in areas like neck, axilla, groin.

• Enclosed by a fibrous capsule.

• Have afferent lymphatic vessels entering the convex side and efferent vessels exiting the hilum.

Microscopic Characteristics:

• Capsule: dense connective tissue with trabeculae extending inward.

• Cortex:

  • Outer cortex: contains lymphoid follicles (with B cells).

  • Germinal centers: sites of B cell activation and proliferation.

  • Paracortex: T cell-rich area, especially around high endothelial venules (HEVs).

• Medulla:

  • Medullary cords: contain B cells, plasma cells, and macrophages.

  • Medullary sinuses: lymph-filled spaces for filtration.

Ultramicroscopic Characteristics:

• Endothelial lining of sinuses is discontinuous, allowing lymphocyte and antigen entry.

• HEVs in paracortex have cuboidal endothelium facilitating lymphocyte migration from blood into the node.

• Reticular fiber network (type III collagen) supports cellular organization.

• Macrophages and dendritic cells present antigens to lymphocytes.

Functional Characteristics:

• Filter lymph to remove pathogens, debris, and cancer cells.

• Site of antigen presentation, B and T cell activation, and immune response initiation.

• Allows recirculation of lymphocytes between blood and lymph.

Tonsils – Principal Structure

General Features:

• Part of MALT (mucosa-associated lymphoid tissue).

• Located at the entrance of the respiratory and digestive tracts (Waldeyer’s ring).

• Not fully encapsulated—partially surrounded by connective tissue capsule.

Principal Structural Components:

• Surface epithelium:

  • Stratified squamous epithelium (palatine and lingual tonsils).

  • Pseudostratified ciliated epithelium (pharyngeal tonsil).

• Crypts: Deep invaginations that trap antigens; especially numerous in palatine tonsils.

• Lymphoid follicles: Contain germinal centers with proliferating B cells.

• Diffuse lymphoid tissue: surrounds follicles, rich in T cells.

Function:

• Acts as the first immune barrier against ingested or inhaled pathogens.

• Initiates local immune responses via antigen capture and lymphocyte activation.

Thymus. Macroscopic and Microscopic Characteristics.

Macroscopic Characteristics:

• Lobulated, bilobed organ located in the superior mediastinum.

• Encapsulated by a thin connective tissue capsule.

• Prominent in infants and children, undergoes involution after puberty, replaced by adipose tissue.

• Has cortex and medulla within each lobule.

Microscopic Characteristics:

• Capsule: connective tissue extends inward as septa, dividing thymus into lobules.

• Cortex:

  • Densely packed with immature T lymphocytes (thymocytes).

  • Contains epithelial reticular cells (ERCs) and macrophages.

• Medulla:

  • Fewer lymphocytes.

  • Contains mature T cells, ERCs, and Hassall’s corpuscles (concentric whorls of ERCs).

Blood-Thymus Barrier.

• Present in the cortex only.

• Composed of:

  • Endothelium of capillaries (tight junctions).

  • Basement membranes of capillaries and ERCs.

  • Macrophages in the perivascular space.

• Function: prevents immature T cells from exposure to circulating antigens during development.

Thymus. Blood and Nerve supply.

Blood Supply:

• Arteries: branches from the internal thoracic, inferior thyroid, and intercostal arteries.

• Capillaries form a dense network in cortex and medulla.

• Veins drain into left brachiocephalic, internal thoracic, and thyroid veins.

Nerve Supply:

• Sympathetic fibers from cervical and upper thoracic ganglia.

• Parasympathetic innervation via the vagus nerve (CN X).

Bone Marrow. Microscopic, Ultramicroscopic and Functional Characteristics.

Microscopic Characteristics:

• Two types:

  • Red marrow: hematopoietically active (in flat bones, vertebrae, epiphyses).

  • Yellow marrow: mostly adipose tissue (in long bones, becomes red in severe anemia).

• Composed of:

  • Hematopoietic cords: developing blood cells.

  • Sinusoidal capillaries: wide, thin-walled vessels for cell entry into bloodstream.

  • Reticular connective tissue (reticular fibers + stromal cells).

Ultramicroscopic Characteristics:

• Stem cells and progenitor cells at various stages of differentiation.

• Sinusoids lined by fenestrated endothelial cells, allowing mature blood cells to pass into circulation.

• Stromal cells (fibroblasts, macrophages, adipocytes) regulate hematopoiesis.

Functional Characteristics:

• Site of hematopoiesis (production of red cells, white cells, and platelets).

• B cell maturation occurs here.

• Stores iron, removes aged erythrocytes via macrophages.

• Can reactivate yellow marrow to red under stress or blood loss.

Spleen. Macroscopic and Microscopic Characteristics.

Macroscopic Characteristics:

• Largest lymphoid organ (~12 cm long).

• Located in the left hypochondriac region, beneath the diaphragm.

• Shaped like a flattened oval; soft and highly vascular.

• Surrounded by a fibrous capsule with trabeculae extending into the parenchyma.

• Has hilum on medial surface:

  • Entry/exit point for splenic artery, vein, lymphatics, and nerves.

Microscopic Characteristics:

• Covered by capsule and trabeculae (dense connective tissue with some smooth muscle).

• Parenchyma divided into:

  1. White Pulp (lymphoid tissue):

  • Surrounds central arterioles.

  • Contains:

    • Periarteriolar lymphoid sheath (PALS): T lymphocytes.

    • Lymphoid follicles/nodules: B lymphocytes, some with germinal centers.

  • Site of immune surveillance and response to blood-borne antigens.

  2. Red Pulp (major portion):

  • Composed of:

    • Splenic cords (cords of Billroth): contain RBCs, macrophages, plasma cells.

    • Sinusoids: wide, irregular capillaries with discontinuous endothelium.

  • Functions in blood filtration, removal of aged/defective RBCs, recycling of iron.

Spleen. Blood and Nerve Supply.

• Artery:

  • Splenic artery (branch of the celiac trunk).

  • Branches into trabecular arteries, then central arterioles within white pulp.

  • Ends in penicillar arterioles, then into sinusoids (red pulp).

• Vein: Splenic vein drains into the portal vein.

• Nerve Supply - Celiac plexus

  • Mostly sympathetic fibers:

    • Regulate vasomotor tone (constriction of blood vessels).

  • No direct parasympathetic innervation of splenic tissue.