Remember that lymphocytes are a key component of adaptive immunity. While all leukocytes originally come from stem cells in the bone marrow, the B lymphocytes (also called B cells) mature in the bone marrow, whereas T lymphocytes (or T cells) mature in the thymus. Together the bone marrow and thymus are the primary lymphoid organs. The secondary lymphoid organs include the lymph nodes, spleen, Peyer’s patches in the small intestine, the appendix, and tonsils. The secondary lymphoid organs are where lymphocytes come into contact with pathogens and are activated. You can think of these secondary lymphoid organs as being like the guard houses and watchtowers along the castle wall. Guards take intruders to the guard house where they are interrogated and the army is called out if necessary. Like the guard stations of a castle, many of these secondary lymphoid organs are also strategically located at sites where invasions are likely. For example, the tonsils guard the nose and mouth from invaders, whereas Peyer’s patches and the appendix guard against invasion from the digestive tract. Secondary lymphoid organs also house macrophages and other immune system cells. The lymph nodes are small oval or bean-shaped secondary lymphoid organs embedded in connective tissue and arrayed along lymphatic vessels. Clusters of lymph nodes are found where several lymphatic vessels converge— for example in the cervical, axillary and inguinal regions. The functions of the lymph nodes are to filter the lymph by removing antigens and other debris that may have entered the lymph, and to enable B and T cells to interact with antigens. These interactions generate immune responses. Here’s a close-up of a lymph node. Lymph nodes are covered by a dense connective tissue capsule. Lymph nodes are separated into sections by bundles of collagen fibers called trabeculae that extend from the capsule deep into the node. Beneath the capsule is the subcapsular sinus. This is the first of a series of sinuses, interconnected dilated channels, through which the lymph flows as it passes through the lymph node. Lymph from the afferent lymphatic vessels empties into the subcapsular sinus and then flows into sinuses in the outer cortex. The outer cortex of the lymph node is the area just below the subcapsular sinus. Here, B cells are found organized into oval-shaped collections of cells called lymphoid follicles. Some of the follicles contain lighter-staining central areas called germinal centers. These are formed by B cells proliferating in response to antigen. Moving inward from the outer cortex, we come to the deep cortex. Lymphocytes exit blood vessels and enter lymph nodes in the deep cortex. T cells congregate in the deep cortex, which is rich with dendritic cells that have captured antigens and are presenting them on their surfaces. T cells wander through the deep cortex searching dendritic cells for that T cell’s special antigen. The central area of the node is the medulla. It is shaped into elongated masses of cells called medullary cords, around which lymph flows. Medullary cords contain both types of lymphocytes as well as macrophages and plasma cells, which are derived from B cells and are antibody-producing factories. Efferent lymphatic vessels leave and blood vessels enter and leave the node at a shallow indentation called the hilum. Let’s observe the flow of lymph through the lymph node. Now let’s look at the spleen. The largest of the lymphoid organs, the spleen is a fist-sized, blood-rich organ located to the left of, and dorsal to, the stomach. The spleen performs the same cleansing function for the blood as the lymph nodes do for the lymph. The spleen removes pathogens and aged erythrocytes and platelets from the blood, stores platelets and breakdown products of red blood cells, and provides a site for the interaction of lymphocytes with antigens. As you explore the structure of the spleen, notice how this structure allows intimate contact between blood and lymphocytes, just as the structure of lymph nodes is designed for intimate contact between lymph and lymphocytes. The mucosal surfaces of the digestive tract, as well as the respiratory tract and genitourinary systems, are vulnerable to invasion by pathogens because they are directly exposed to the external environment. Like guard houses and watchtowers strung along a castle wall, we have collections of lymphoid tissue, called the mucosa-associated lymphoid tissues (MALT), strategically distributed throughout the mucosae. MALT includes the tonsils, appendix, and Peyer’s patches of the small intestine, as well as more diffuse collections of cells in the respiratory tracts and other mucosae. These tissues are unencapsulated or partially encapsulated collections of lymphocytes. MALT contains both B and T cells, with the B cells occurring in lymphoid follicles similar to those found in lymph nodes and the spleen. Peyer’s patches are found in the mucosa of the distal portion of the small intestine. In this photomicrograph you can see the many lymphoid follicles in the small intestine that make up the Peyer’s patches. Like the tonsils and other MALT components, Peyer’s patches are located where they can sample the antigens moving through hollow organs open to the external environment. If a pathogen escapes the defenses of the MALT, it can still be cleared by responses of the lymph nodes or spleen. We have now finished our investigation of MALT, the last of the secondary lymphoid tissues and organs that we will consider. Now, let’s look at the thymus, a primary lymphoid organ. The thymus is the site for differentiation of lymphocytes into mature T cells. Thymic hormones and other factors influence the development of immature T cells. The thymus is a bilobed organ located in the mediastinum. In young children, the thymus is large relative to body size. The relative size of the thymus, as well as its function, gradually decreases with age. In the elderly, thymic epithelial cells are almost entirely replaced by fat cells and fibrous connective tissue. This process, called thymic atrophy, may be one reason why the elderly are more susceptible to infection. Let’s observe thymic atrophy. Now we’ll view the organ more closely. Each lobe of the thymus is divided into many lobules. Each lobule contains an outer cortex… and an inner medulla. Most of the cells in the thymus are immature T cells at various stages of development. Scattered amongst the T cells are thymic epithelial cells which influence T cell development and secrete thymic hormones such as thymopoetin and the thymosins. Within the medulla are distinctively shaped structures called thymic corpuscles. thymic corpuscles are clusters of keratinized epithelial cells with a whorled appearance that are scattered throughout the medullary area. While their function is not completely understood, they are thought to be involved in the development of a type of T cell called a regulatory T cell.
Table of contents
- 1. Introduction to Anatomy & Physiology5h 40m
- What is Anatomy & Physiology?20m
- Levels of Organization13m
- Variation in Anatomy & Physiology12m
- Introduction to Organ Systems27m
- Homeostasis9m
- Feedback Loops11m
- Feedback Loops: Negative Feedback19m
- Feedback Loops: Positive Feedback11m
- Anatomical Position7m
- Introduction to Directional Terms3m
- Directional Terms: Up and Down9m
- Directional Terms: Front and Back6m
- Directional Terms: Body Sides12m
- Directional Terms: Limbs6m
- Directional Terms: Depth Within the Body4m
- Introduction to Anatomical Terms for Body Regions3m
- Anatomical Terms for the Head and Neck8m
- Anatomical Terms for the Front of the Trunk8m
- Anatomical Terms for the Back9m
- Anatomical Terms for the Arm and Hand9m
- Anatomical Terms for the Leg and Foot15m
- Review- Using Anatomical Terms and Directions12m
- Abdominopelvic Quadrants and Regions19m
- Anatomical Planes & Sections17m
- Organization of the Body: Body Cavities13m
- Organization of the Body: Serous Membranes14m
- Organization of the Body: Serous Membrane Locations8m
- Organization of the Body: Thoracic Cavity8m
- Organization of the Body: Abdominopelvic Cavity12m
- 2. Cell Chemistry & Cell Components12h 37m
- Atoms- Smallest Unit of Matter57m
- Isotopes39m
- Introduction to Chemical Bonding19m
- Covalent Bonds40m
- Noncovalent Bonds5m
- Ionic Bonding37m
- Hydrogen Bonding19m
- Introduction to Water7m
- Properties of Water- Cohesion and Adhesion7m
- Properties of Water- Density8m
- Properties of Water- Thermal14m
- Properties of Water- The Universal Solvent17m
- Acids and Bases12m
- pH Scale21m
- Carbon8m
- Functional Groups9m
- Introduction to Biomolecules2m
- Monomers & Polymers11m
- Carbohydrates23m
- Proteins25m
- Nucleic Acids34m
- Lipids28m
- Microscopes10m
- Prokaryotic & Eukaryotic Cells26m
- Introduction to Eukaryotic Organelles16m
- Endomembrane System: Protein Secretion34m
- Endomembrane System: Digestive Organelles15m
- Mitochondria & Chloroplasts21m
- Endosymbiotic Theory10m
- Introduction to the Cytoskeleton10m
- Cell Junctions8m
- Biological Membranes10m
- Types of Membrane Proteins7m
- Concentration Gradients and Diffusion9m
- Introduction to Membrane Transport14m
- Passive vs. Active Transport13m
- Osmosis33m
- Simple and Facilitated Diffusion17m
- Active Transport30m
- Endocytosis and Exocytosis15m
- 3. Energy & Cell Processes10h 7m
- Introduction to Energy15m
- Laws of Thermodynamics15m
- Chemical Reactions9m
- ATP20m
- Enzymes14m
- Enzyme Activation Energy9m
- Enzyme Binding Factors9m
- Enzyme Inhibition10m
- Introduction to Metabolism8m
- Redox Reactions15m
- Introduction to Cellular Respiration22m
- Types of Phosphorylation11m
- Glycolysis19m
- Pyruvate Oxidation8m
- Krebs Cycle16m
- Electron Transport Chain14m
- Chemiosmosis7m
- Review of Aerobic Cellular Respiration19m
- Fermentation & Anaerobic Respiration23m
- Introduction to Cell Division22m
- Organization of DNA in the Cell17m
- Introduction to the Cell Cycle7m
- Interphase18m
- Phases of Mitosis48m
- Cytokinesis16m
- Cell Cycle Regulation18m
- Review of the Cell Cycle7m
- Cancer13m
- Introduction to DNA Replication22m
- DNA Repair7m
- Central Dogma7m
- Introduction to Transcription20m
- Steps of Transcription19m
- Genetic Code25m
- Introduction to Translation30m
- Steps of Translation23m
- Post-Translational Modification6m
- 4. Tissues & Histology10h 3m
- Introduction to Tissues & Histology16m
- Introduction to Epithelial Tissue24m
- Characteristics of Epithelial Tissue37m
- Structural Naming of Epithelial Tissue19m
- Simple Epithelial Tissues1h 2m
- Stratified Epithelial Tissues55m
- Identifying Types of Epithelial Tissue32m
- Glandular Epithelial Tissue26m
- Introduction to Connective Tissue36m
- Classes of Connective Tissue8m
- Introduction to Connective Tissue Proper40m
- Connective Tissue Proper: Loose Connective Tissue56m
- Connective Tissue Proper: Dense Connective Tissue49m
- Specialized Connective Tissue: Cartilage44m
- Specialized Connective Tissue: Bone12m
- Specialized Connective Tissue: Blood9m
- Introduction to Muscle Tissue7m
- Types of Muscle Tissue45m
- Introduction to Nervous Tissue8m
- Nervous Tissue: The Neuron8m
- 5. Integumentary System2h 20m
- 6. Bones & Skeletal Tissue2h 16m
- An Introduction to Bone and Skeletal Tissue18m
- Gross Anatomy of Bone: Compact and Spongy Bone7m
- Gross Anatomy of Bone: Periosteum and Endosteum11m
- Gross Anatomy of Bone: Bone Marrow8m
- Gross Anatomy of Bone: Short, Flat, and Irregular Bones5m
- Gross Anatomy of Bones - Structure of a Long Bone23m
- Microscopic Anatomy of Bones - Bone Matrix9m
- Microscopic Anatomy of Bones - Bone Cells25m
- Microscopic Anatomy of Bones - The Osteon17m
- Microscopic Anatomy of Bones - Trabeculae9m
- 7. The Skeletal System2h 35m
- 8. Joints2h 17m
- 9. Muscle Tissue2h 33m
- 10. Muscles1h 11m
- 11. Nervous Tissue and Nervous System1h 35m
- 12. The Central Nervous System1h 6m
- 13. The Peripheral Nervous System1h 26m
- Introduction to the Peripheral Nervous System5m
- Organization of Sensory Pathways16m
- Introduction to Sensory Receptors5m
- Sensory Receptor Classification by Modality6m
- Sensory Receptor Classification by Location8m
- Proprioceptors7m
- Adaptation of Sensory Receptors8m
- Introduction to Reflex Arcs13m
- Reflex Arcs15m
- 14. The Autonomic Nervous System1h 38m
- 15. The Special Senses2h 41m
- 16. The Endocrine System2h 48m
- 17. The Blood1h 22m
- 18. The Heart1h 42m
- 19. The Blood Vessels3h 35m
- 20. The Lymphatic System3h 16m
- 21. The Immune System14h 37m
- Introduction to the Immune System10m
- Introduction to Innate Immunity17m
- Introduction to First-Line Defenses5m
- Physical Barriers in First-Line Defenses: Skin13m
- Physical Barriers in First-Line Defenses: Mucous Membrane9m
- First-Line Defenses: Chemical Barriers24m
- First-Line Defenses: Normal Microbiota7m
- Introduction to Cells of the Immune System15m
- Cells of the Immune System: Granulocytes28m
- Cells of the Immune System: Agranulocytes26m
- Introduction to Cell Communication5m
- Cell Communication: Surface Receptors & Adhesion Molecules16m
- Cell Communication: Cytokines27m
- Pattern Recognition Receptors (PRRs)48m
- Introduction to the Complement System24m
- Activation Pathways of the Complement System23m
- Effects of the Complement System23m
- Review of the Complement System13m
- Phagocytosis17m
- Introduction to Inflammation18m
- Steps of the Inflammatory Response28m
- Fever8m
- Interferon Response25m
- Review Map of Innate Immunity
- Introduction to Adaptive Immunity32m
- Antigens12m
- Introduction to T Lymphocytes38m
- Major Histocompatibility Complex Molecules20m
- Activation of T Lymphocytes21m
- Functions of T Lymphocytes25m
- Review of Cytotoxic vs Helper T Cells13m
- Introduction to B Lymphocytes27m
- Antibodies14m
- Classes of Antibodies35m
- Outcomes of Antibody Binding to Antigen15m
- T Dependent & T Independent Antigens21m
- Clonal Selection20m
- Antibody Class Switching17m
- Affinity Maturation14m
- Primary and Secondary Response of Adaptive Immunity21m
- Immune Tolerance28m
- Regulatory T Cells10m
- Natural Killer Cells16m
- Review of Adaptive Immunity25m
- 22. The Respiratory System3h 20m
- 23. The Digestive System2h 5m
- 24. Metabolism and Nutrition4h 0m
- Essential Amino Acids5m
- Lipid Vitamins19m
- Cellular Respiration: Redox Reactions15m
- Introduction to Cellular Respiration22m
- Cellular Respiration: Types of Phosphorylation14m
- Cellular Respiration: Glycolysis19m
- Cellular Respiration: Pyruvate Oxidation8m
- Cellular Respiration: Krebs Cycle16m
- Cellular Respiration: Electron Transport Chain14m
- Cellular Respiration: Chemiosmosis7m
- Review of Aerobic Cellular Respiration18m
- Fermentation & Anaerobic Respiration23m
- Gluconeogenesis16m
- Fatty Acid Oxidation20m
- Amino Acid Oxidation17m
- 25. The Urinary System2h 39m
- 26. Fluid and Electrolyte Balance, Acid Base Balance Coming soon
- 27. The Reproductive System2h 5m
- 28. Human Development1h 21m
- 29. Heredity Coming soon
20. The Lymphatic System
Lymphoid Cells & Tissues
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