BIO U4 AOS1

• Pathogens


  Biological agents of infectious diseases
• Disease


  Causes organism to not be at ease, causes negative effect on the structure or function of all or part of an organism
• Barriers to infection


  • Physical
  • Chemical
  • Microbiological
• Physical barriers


  • Skin or bark
  • Epithelial cells prevent pathogens from entering organism
  • Cell walls provide strength and flexibility
  • Cutin and waxes make up cuticle, thicker cuticle prevents more pathogens from infecting
• Chemical barriers


  • Lysozyme enzymes and toxic metabolites
  • Fluid in lungs contains proteins that act as surfactants
  • Alkaloids like caffeine and nicotine are toxic to many organisms
  • Cyanogenic glycosides disrupt ATP production
  • Stomach acid kills microorganisms
  • Phenolics like phytoalexins and tannins have antibiotic properties
• Microbiological barriers


  • Non-pathogenic normal flora compete with pathogens for space and resources
  • Antibiotics disrupt normal gut flora and predispose to infections
• How pathogens breach first line of defence

  1. Faced with attacking cells and molecules

  2. Must distinguish self from non-self

  3. Innate immune response identifies pathogen-associated molecular patterns (PAMPs)
• Innate immune response


  • Pre-existing defence against invasions by pathogens
  • Non-specific, rapid, fixed responses, does not result in immunological memory
• Innate immune response in plants

  • Chemical response triggered by recognition of PAMPs
  • Possess resistance genes that switch on defences when PAMPs are recognised
  • Produce proteins and enzymes for defence against pathogens
• Adaptive immune response


  Antigen-specific and results in immunological memory
• Immunological memory


  Ability of lymphocytes to remember antigens after primary exposure and respond more rapidly
• Cells of the immune system

  • Granulocytes
  • Phagocytes
  • Natural killer cells
• Granulocytes


  • Release chemicals and enzymes from granules
  • Neutrophils, basophils, eosinophils, mast cells
• Phagocytes


  • Eat foreign material, cellular debris or remnants of dead cells
  • Move to point of pathogen entry and engulf/break down pathogens
  • Neutrophils, macrophages, dendritic cells
• Natural killer cells


  • Attack cells with fewer MHC 1 proteins on surface
  • Detect infected cells, create holes in membrane and release enzymes to program cell death
• Inflammatory response


  1. Reaction to infection, increased blood flow and vasodilation

  2. Acute inflammation involves phagocytes, chronic involves lymphocytes
• Antigens


  Molecules or parts of molecules that stimulate immune response
• Self-antigens


  Identify immune cells as self and are ignored
• Non-self antigens


  Stimulate immune response
• Allergens


  Antigens that result in immediate hypersensitivity reactions and allergies
• MHC markers/HLA


  Proteins on surface of body's cells that aid antigen recognition
• Cell-mediated immune response


  Through cytotoxic T cells
• Antigens


  • Most are protein based and composed of one or more polypeptide chains
  • Can also be composed of carbohydrates, lipids and even nucleic acids e.g. human ABO blood group
• Self-antigens


  Identify immune cell as self (ignore)
• Non-self antigens


  Immune system responds to
• Distinguishing self from non-self

  Role of cell surface receptors
• Cell surface receptors


  For self-antigens<|>For foreign antigens
• Antigen recognition


  Depends on the detection of antigens by the receptors
• Types of antigens


  • Expressed/presented on surface of membrane where they act as recognition sites for immune system
  • Not all attached to cell e.g. toxins released by bacteria, circulate freely in body fluids
• Antigens that result in immediate hypersensitivity reactions

  Allergens<|>Allergies (rapid overreaction of immune system to antigens)
• Responding to antigens


  Antigen recognition dependent on the detection of antigens by receptors
• MHC markers OR HLA


  Proteins on surface of your body's cells that present self-antigens or non-self antigens to T lymphocytes
• MHC markers


  • Self antigens - cells move around with these markers
  • When fail to recognise self-cells, autoimmune disease can result - attack own cells
• Classes of MHC


  • Class 1 - all nucleated cells (no RBC)
  • Class 2 - specific immune cells (B or T lymphocytes)
• Two stage process of selecting T lymphocytes

  1. Positive selection - T lymphocytes that do not interact with MHC proteins are destroyed by apoptosis

  2. Negative selection - T lymphocytes that react with self-antigens in thymus bind tightly to the cells and die
• Self-tolerance


  Inability to respond to self antigens - autoimmune diseases
• Immunogens


  Antigens that elicit an immune response
• Antigen presentation


  Some intracellular pathogens down-regulate MHC-1 expression to avoid recognition by cytotoxic T cells, and natural killer cells appear to have evolved in response to this
• Natural killer cells


  Recognise absence of MHC-1 in infected and damaged cells, and respond by releasing molecules that destroy the cell
• Types of immune response

  • Cell-mediated response (T cells)
  • Humoral response (primary - B cells)
  • Humoral response (secondary - B cells)