Every cell in our body has a unique set of proteins on its surface that mark a cell as belonging to self, in other words, one’s own body. One major group of these self-antigens is called the MHC proteins. These proteins cradle antigens for presentation to T cells. Remember that T cells do not recognize free antigens, but instead, recognize antigens combined with an MHC protein. MHC stands for major histocompatibility complex. Histocompatibility simply refers to tissue compatibility. MHC proteins determine whether a transplanted tissue or organ will be accepted as self, or will be rejected following transplantation. Before any donor organ is transplanted into a recipient, the donor and recipient’s MHC proteins are matched as closely as possible. Matching is difficult because there are millions of different combinations of MHC that could be expressed in an individual. This makes it unlikely that any two people will have exactly the same set of MHC proteins unless they are identical twins. You’ve learned that cellular immunity is directed against intracellular pathogens. Cytotoxic T cells are activated CD8 cells that attack our own cells if they have become infected by an intracellular pathogen, most often a virus, or if they have become cancerous. Cytotoxic T cells circulate through the body, crawling over cells to examine the self-antigens on their surfaces. How can a T cell know there is something wrong inside a cell by examining the surface of that cell? The answer is class 1 MHC proteins, which continuously bring bits and pieces of endogenous proteins, that is, proteins from inside the cell, to the surface for inspection. Normal cell proteins are continuously degraded in the cytoplasm. Special transporters bring these protein fragments into the endoplasmic reticulum where they are loaded onto newly synthesized class 1 MHC proteins. Now let’s watch the loaded class 1 MHC protein transport it to the surface of the cell. Keep in mind that the process you just saw happens continuously. It is the way of randomly sampling proteins in the cytoplasm and displaying their fragments on the cell’s surface. Also remember that during their education in the thymus, T cells have been taught to ignore self-antigens on MHC. Now watch the cytotoxic T cell ignore this self-antigen. Observe a virus infecting the cell below it. The viral protein is loaded onto class 1 MHC and transported to the cell surface. Here, the cytotoxic T cell recognizes the foreign antigenic peptide. Only a few kinds of cells have class 2 MHC proteins. These are the antigen-presenting cells: dendritic cells, macrophages, and B cells. These cells are responsible for communicating with CD4 cells, which are destined to become, or have already become, helper T cells. The antigens that are presented on class 2 MHC proteins are exogenous antigens, that is, they originate from outside the cell. Exogenous antigens originate outside the antigen-presenting cell, usually as pathogens such as bacteria. Let’s follow an exogenous antigen on its way to being displayed on a class II MHC protein. Let’s watch the dendritic cell phagocytose the bacterium. The next step of the process is the same as for macrophages discussed earlier. The phagosome fuses with the lysosome and the dead bacteria are digested into small fragments in the phagolysosome. Class 2 MHCs are produced in the rough endoplasmic reticulum and are exported to the plasma membrane. On the way, they fuse with the phagolysosome, allowing the fragmented exogenous antigens to be loaded onto the class 2 MHC. Presenting antigen on class 2 MHC proteins has two main outcomes: First, dendritic cells and macrophages present antigen to CD4 cells, triggering them to become helper T cells. Second, B cells and macrophages present antigen to already-activated helper T cells to request further activation that will enhance their own effectiveness. Class 1 and class 2 MHC proteins are structurally distinct molecules, but both serve the function of displaying antigens on the surface of a cell. Let’s review the differences between MHC 1 and MHC 2 proteins. MHC 1 proteins are found on the surface of virtually all nucleated body cells, while MHC II proteins are less widespread and found only on the surfaces of APCs such as dendritic cells, macrophages and B cells. CD4 cells (or helper T cells) recognize Class recognize Class 2 MHC proteins. WhileCD 8 cells, or cytotoxic T cells, recognize Class 1 MHC Proteins. Remember that normally occurring proteins are displayed on the surfaces of all cells. However, MHC 1 and MHC 2 can also display foreign antigens. Where do these foreign antigens come from for each class of MHC? MHC 1 proteins display endogenous antigens, fragments of proteins synthesized inside the cell including intracellular pathogens or proteins made by cancer cells. However, MHC 2 proteins display exogenous antigens or phagocytized extracellular pathogens. Now imagine that the cell displaying the foreign antigen is saying something to the T cell. What is its message? The MHC I protein’s message is “I belong to self, but have been invaded or BECOME cancerous. Kill me! The MCH 2 protein is saying “I belong to self, but have captured an alien invader. This what it looks like. Help me mount a defense against it. Note that there is one exception to our generalized messages here, which occurs during activation of CD8 cells.
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
21. The Immune System
Classes of Antibodies
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