Lymphatic System and Immunity Vocabulary

Lymphatic System and Immunity Vocabulary

• Immunity: The ability of an organism to resist infection.
• Antigen: A molecule capable of generating an immune response.
• Natural Killer Cells: White blood cells that recognize and destroy infected or abnormal cells.
• Antibody: A protein produced by B cells that interacts with a specific antigen.
• Pyrogen: A chemical that causes an increase in body temperature (fever).
• Phagocytosis: The process by which cells engulf and remove pathogens or debris.
• Cytokines: Small proteins that cause changes in other cell types, stimulating immune responses.
• Antigen-Presenting Cells (APCs): Cells that display antigens to T cells, such as macrophages and dendritic cells.
• Class I Cells: Nucleated cells that present antigens via MHC Class I molecules, indicating infection.
• Class II Cells: Antigen-presenting cells (APCs) that present antigens via MHC Class II molecules to activate helper T cells.

Specific vs. Non-Specific Defenses

• Non-Specific Defenses: Attack any pathogen, including:
  • External Barriers (skin, mucous membranes)
  • Phagocytic Cells (macrophages, dendritic cells)
  • Natural Killer Cells
  • Fever
  • Inflammation
• Specific Defenses: Attack a specific, identified pathogen, resulting in future protection (immunity).
  • Cell-mediated immunity
  • Antibody-mediated immunity

Relationship and Function of Immune Cells

• Infected Body Cell: Presents antigens via MHC Class I to activate cytotoxic T cells, leading to its destruction.
• Macrophages: Engulf pathogens and debris, present antigens via MHC Class II to activate helper T cells, and participate in phagocytosis.
• Natural Killer Cells: Recognize and destroy infected or abnormal cells.
• Helper T Cells (TH): Activated by antigen-presenting cells (APCs), secrete cytokines to stimulate other immune cells (cytotoxic T cells and B cells), and are required for all immune responses.
• Cytotoxic T Cells (TC): Activated by helper T cells and infected cells, kill infected cells displaying specific antigens via MHC Class I.
• B Cells: Activated by helper T cells and antigens, differentiate into plasma cells and memory B cells.
• Plasma Cells: Produce and secrete antibodies.
• Memory (T and B) Cells: Provide a faster response upon subsequent encounters with the same antigen.

Cell-Mediated vs. Antibody-Mediated Immunity

• Cell-Mediated Immunity: Involves cytotoxic T cells directly killing infected cells.
• Antibody-Mediated Immunity: Involves B cells producing antibodies that target pathogens.

Antigen-Presenting Cells

• Class I MHC: Found on all nucleated cells; present antigens to cytotoxic T cells when the cell is infected, signaling "HELP – I’M INFECTED!!! KILL ME!!!"
• Class II MHC: Found on antigen-presenting cells (APCs); present antigens to helper T cells, signaling "HEY LOOK WHAT I FOUND. IT’S A FOREIGN ANTIGEN. FIND AND DESTROY!!!"

Learning Objectives

• Compare and contrast non-specific and specific defense mechanisms.
• Generally describe the roles of the types of white blood cells that we discussed in class (natural killer cells, T-cells, B-Cells, macrophages, dendritic cells)
• Describe the barriers that help prevent pathogens from entering the body.
• Define phagocytosis and why it is important for the non-specific and specific defenses of the body.
• Compare and contrast antibody-mediated and cell-mediated immunity.
• Explain the difference between an antigen and an antibody.
• Compare and contrast Class I MHC (infected cells) and Class II MHC (antigen- presenting cells) cell types and their roles in the immune system.
• Explain the role of antigen-presenting cells in both specific and non-specific defenses.
• Define cytokines and their role in the immune system.
• With respect to T cells (helper and cytotoxic) and B cells (plasma cells):
  • Describe how each cell type is activated.
  • Describe the function of each cell type after activation.
  • Describe the role of memory cells produced after activation.
  • Predict the outcome if any cell type fails to function.
• Outline the role of antibodies and their major effect on invading pathogens
• Analyze how vaccinations help achieve immunity.

Major Immunity Questions

• How do pathogens enter our body?
• How does our body “recognize” a pathogen?
• What is the difference between non-specific and specific immunity?
• How are immune cells activated?
• What are antibodies and how do they help with immunity?

Lymph Flow

• Interstitial fluid enters lymphatic capillaries, is filtered by white blood cells in lymph nodes, and transported back to the bloodstream at the left and right subclavian veins.

Lymph Nodes

• Lymph nodes house white blood cells that help with immunity.
• Lymph enters the node and travels through sinuses where white blood cells are located.
• Capillaries provide oxygen and nutrients for the cells.

Pathogens

• Pathogens are varied and cause disease/infection in a host organism, leading to an immune response.
  • Cellular (Living) Pathogens
    • Viruses: Multi-celled but can only reproduce inside a plant, animal, or person.
    • Bacteria: Tiny one-celled creatures that can live inside or outside the body.
    • Fungi: Multi-celled but plant-like, taking nutrition from a plant, tree, or animal.
    • Parasites: Actual complex living organisms that can live in the intestinal tract or bloodstream.
    • Protozoa: One-celled creatures usually spread through water.
  • Acellular (Non-living)
    • Virus
    • Prion: Multi-celled but can only reproduce inside a plant, animal, or person.

Nonspecific vs. Specific Defenses

• Nonspecific Defenses:
  • External Barriers: Integumentary system (skin, dryness, acidity), mucous membranes (trap pathogens).
  • Phagocytic Cells: Remove pathogens by engulfing them.
  • Natural Killer Cells: Recognize and destroy certain abnormal cells.
  • Fever: Increase in body temperature caused by pyrogens.
  • Inflammation
• Specific Defenses: Target a single pathogen and result in future protection (immunity).
  • Cell-mediated immunity
  • Antibody-mediated immunity

Antigens

• Molecules that have the potential to generate antibodies (create an immune response).
• Most commonly proteins or carbohydrates found in the plasma membrane, envelope (virus), or cell wall of pathogens.
• Antibodies (proteins) only interact with specific antigens.

Self-Antigens

• “Self-antigens” also exist but normally do not trigger a response.

Nonspecific Defenses

• Mechanisms that respond to all pathogens in the same way.
  • External barriers: Integumentary system (skin): dryness, acidity. Mucous membranes: trap pathogens.
  • Phagocytic cells: Remove pathogens by engulfing them. Includes dendritic cells and macrophages, which are critical to specific immunity because they present antigens they engulf.

Phagocytosis and Antigen Presentation

1. A bacterium is engulfed by phagocytosis into a dendritic cell and is encased in a phagosome.
2. Lysosomes fuse with the phagosome and digest the bacterium.
3. Immunodominant epitopes are associated with MHC II and presented on the cell surface.

Natural Killer Cells

• Recognize certain types of abnormal antigens presented by cells infected by viruses or by abnormal cells (e.g., cancer).
• Results in destruction of the infected cell.

Fever

• Increase in body temperature caused by pyrogens (chemicals) released during phagocytosis.
• Results in brain “resetting” body’s thermostat.
• Increases metabolism to speed up immune response.
• Fevers are a natural response to infection and may be beneficial.

What Happens When a Pathogen Enters the Body?

• Pathogen enters extracellular space and gets sucked into lymph vessels, then travels to lymph nodes and can interact with B cells.
• Pathogen is engulfed by macrophages at the site of injury.
• Pathogens “trick” our cells and gain entry into the cytoplasm, then use cells to make more pathogens = infected cells.

Specific Defenses

• Target a single pathogen and result in future protection (immunity).
  • Cell-mediated immunity: Antigen-presenting infected cells activate and are killed by cytotoxic T-cells.
  • Antibody-mediated immunity: Antigens activate the production of antibodies by B-cells.
• Two systems above are coordinated and require Helper T-cells.
• Helper T-cells are activated by “professional” antigen-presenting cells such as dendritic cells and macrophages.

Antigen Presentation

• Both infected cells and “professional” antigen-presenting cells “show” or “present” antigens to cells of the immune system (T cells).
• Antigens can be self or foreign.
• Presented on 1 of 2 types of MHC proteins: Class I and Class II.
• Although both classes present antigens, they use different proteins that affect the type of cell that can recognize the antigen.

MHC – Major Histocompatibility Complex

• MHC – family of protein complexes specialized for presenting antigens.
  • MHC Class I: found on all nucleated cells. This class presents antigens when the cell reports "HELP – I’M INFECTED!!! KILL ME!!!”
  • MHC Class II: found only on antigen-presenting cells (APCs). This class presents antigens when the cell reports “HEY LOOK WHAT I FOUND. IT’S A FOREIGN ANTIGEN. FIND AND DESTROY!!!”
• “Professional” APCs:
  • Macrophages
  • Dendritic cells

Activation of Helper T Cells

• This step is required to activate ALL immune responses.
• Start with inactive TH (helper) cell. Inactive TH cell binds antigen presented by class II cells (professional APCs).
• Activated TH cell divides.
• Activated TH cell secretes cytokines, which are small proteins that cause changes in many cell types and help stimulate TC cells and B cells and non-specific responses.
• Activation also causes differentiation of memory TH cells, which are stored in lymph nodes for a faster response with the next encounter of the same antigen.

Activation of Cytotoxic T Cells

• Occurs at the same time TH cells are being activated but requires TH cells for full activation.
• Initiates cell-mediated immunity à TC cells killing infected cells.
• Start with inactive TC cell. Inactive Tc cell binds antigen presented by class I cells (infected cells). Further activated by cytokines produced by TH cells.
• Activated TC divides to create a specific army of cells.
• Activated TC cells attack infected cells (class I) with the same presented antigen, resulting in cell lysis (breaking open).
• Activation also causes differentiation of memory TC cells, which are stored in lymph nodes for a faster response with the next encounter of the same antigen.

B Cell Activation

• Happens at the same time as other responses.
• B cells have antibody receptors for specific antigens on the cell surface.
• Sensitization à Receptor binds antigen, engulfs, and presents on MHC II.
• Activation à Bind TH cell activated with the same antigen, “Co-stimulation”. This is a required step to prevent an accidental activation of B cells.
• Produce plasma cells à make and secrete antibodies.
• Produce memory B cells à faster response if exposed to antigen again.

Antibodies

• Have several modes of action:
  • Neutralization à bind to part of antigen that is toxic.
  • Agglutination à antibodies bind to multiple antigens causing them to stick together and prevents transport throughout the body.
  • Improved phagocytosis due to the presence of bound antibodies.
  • Precipitation of antigens from body fluids.

Specific Resistance Recap

• Specific response occurs in lymph nodes, which get swollen when you are sick.
• Immunity creates a memory of disease, leading to a quick response if presented with the same antigen because of memory T and B cells in both the cellular and the antibody response.
• Vaccines also generate immunological memory, stimulating the immune response using dead antigen or attenuated viruses, so T and B memory cells are produced so if faced with the same disease your immune system launches a fast attack. The vaccines generate the same benefits as memory cells.