Immune System Histology PDF

Immune/Lymphatic system Functions ➔ Distinguishes between bodily objects and foreign objects. ➔ Breakdown of pathogens: parasitic worms, fungi, protozoa, bacteria and viruses. ➔ Detects & kills foreign bodies: virus-infected cells, cancer cells, cells of transplanted organs etc. ➔ R...

Immune/Lymphatic system Functions ➔ Distinguishes between bodily objects and foreign objects. ➔ Breakdown of pathogens: parasitic worms, fungi, protozoa, bacteria and viruses. ➔ Detects & kills foreign bodies: virus-infected cells, cancer cells, cells of transplanted organs etc. ➔ Removal of dead cells & other debris. ➔ Drains interstitial fluid ➔ Transports dietary lipids Content 1. Lymphatic vessels Simple squamous epithelium Takes up interstitial fluid from capillaries Low pressure/doesn’t pump 2. Lymphatic fluids Fluid in lymph vessels Similar to interstitial fluid Lymphatic capillary ↓ Lymphatic vessel ↓ Lymph node ↓ Lymphatic vessel ↓ Lymphatic trunk ↓ Collecting duct: Thoracic duct & Right lymphatic duct ↓ Subclavian vein ↓ Heart 3. Lymphatic organs MALT Primary lymphoid organs: No contact with foreign bodies, maturation of lymphocytes into mature cells ➔ Thymus - t-lymphocyte maturation ➔ Bone marrow - b-lymphocyte maturation Secondary lymphoid organs: Contact with foreign bodies, lymphocytes undergo immune response ➔ Lymph node - react to antigens in lymph fluid ➔ Spleen - react to antigens in blood ➔ Tonsils - react to antigens in air Primary & secondary follicles 1. Primary follicles: inactive, absence of immune response, contains inactive B-lymphocytes. Under the microscope: - Dark densely packed B-lymphocytes 2. Secondary follicles: active, presence of immune response, active B-lymphocytes. Under the microscope: - Mantle: Dark outer layer (aggregation of inactive B lymphocytes) - Germinal centre: Light inner layer (dividing B lymphocytes, macrophages, plasma cells & follicular dendritic cells) - Paracortex - Adjacent dark layer (T cells, dendritic cells, macrophages & high endothelial venules) Immune cells & function Cell type Cell structure Nucleus Function Lymphocytes: Active 1. Small Round Specific immune B lymphocytes 2. Little cytoplasm Hyperchromatic response. T lymphocytes Nucleolus invisible Reacts to one antigen only. Inactive 1. Large Round & large Develops plasma cells. 2. More cytoplasm Vesicular T-helper cells, cytotoxic Nucleolus visible cells etc. Macrophages Large Large nucleus Phagocytosis of foreign Develop from monocytes Large nucleolus bodies & damaged Many lysosomes (phagocytic) apoptotic cells. “Coloured bodies” Specific immune response Plasma cells Develop form B-lymphocytes. Round & small Produce & secrete Larger than active Heterochromatin (immunoglobulins) B-lymphocytes. appears as a clock antibodies during Basophilic cytoplasm face/wagon wheel. specific immune Nucleolus visible. response. Dendritic cells Large Large nucleus Antigen-presenting cells. Few lysosomes Take up foreign antigens Long branched cell processes & present them to Large surface for interaction with T-lymphocytes. antigens. Basic structure of lymphoid organs 1. Capsule: Dense irregular connective tissue 2. Septa/trabeculae: Dense irregular connective tissue 3. Reticular fibres: Collagen III connective tissue & fibroblasts in all connective tissue. Structures 1. Thymus Function: Lymphocyte precursors develop into immunocompetent T-lymphocytes. Proliferation via mitosis → Maturation of T-cells → Elimination of self-reactive cells Structure Capsule No afferent lymph vessels Connective tissue No sinuses Septa Cortex Medulla with Hassall corpuscles 2. Lymph Nodes Function: Filters & removes foreign particles from lymph fluid, provides specific immune response to foreign antigens in lymph fluids. Structure: Capsule Subcapsular sinuses: ○ Afferent lymphatic vessels Trabeculae/Septa Cortex: ○ Follicles with mainly B lymphocytes, few T-lymphocytes, macrophages & APCs Paracortex: ○ T-lymphocytes & macrophages Medulla: ○ No follicles ○ Medullary cords (with B-lymphocytes, macrophages & plasma cells) ○ Medullary sinuses Pathway of fluid in a lymph node: Afferent vessel → Subcapsular sinus → Cortical sinuses → Paracortical sinus → Medullary sinus → Efferent vessel Non-specific immune response Specific immune response 1. Lymph enters nodes via B-Lymphocytes T-lymphocytes afferent vessels. 1. B-cells enter lymph 1. T-cells enter lymph 2. Lymph flows through node via HEV, travels nodes via HEV. sinuses into medulla. into outer cortex 2. In the paracortex, the 3. Lymph leaks out and 2. In follicles, the b-cell is T-cell is activated & spreads through activated & proliferates proliferates if in contact medulla. if in contact with the with the correct antigen. 4. Foreign particles are correct antigen. 3. Moves out of lymph engulfed by 3. B-cells move to medulla nodes via efferent macrophages & APCs and mature into plasma vessels. 5. Lymph leaves nodes via cells & produce 4. Travels to locations efferent vessels. antibodies. where it is needed. 6. Eventually, lymph joins 4. Antibodies secreted into the bloodstream in the lymph, thus into the subclavian vein towards bloodstream. the heart. 3. Tonsils Function: specific immune response to foreign bodies into the oral cavity & nose. Palatine structure: Epithelial layer on luminal surface ○ Keratinised stratified squamous epitheliumConnective tissue septa Crypts formed by epithelial layer Glands ○ Secretes mucus No afferent lymph vessels Cortex No sinuses ○ Follicles: B-lymphocytes ○ T-lymphocytes, APCs & macrophages ○ High endothelial venules Medulla Capsule on non-luminal part Lingual tonsil structure: Epithelial layer on luminal surface ○ Keratinised stratified squamous epithelium Lamina propria Follicles One crypt 4. Lymph tissue/MALT - Small accumulation of lymphoid tissue present below epithelium in certain organs. Function: specific immune response against antigens that penetrate the mucosa. Forms: BALT (bronchus), GALT (gut), SALT (skin).