Chapter 5

• Antigens

  Molecules that elicit an immune response

  
• Antigens

  • Play crucial role in immunity
  • Can be recognised by T lymphocytes or by antibodies produced by B lymphocytes
  • Importance = recognise potentially harmful pathogens and mount an immune response against them

  
• Antibodies and antigens

  Work as a lock and key
• Structure of antigens

  • Protein-based (composed of one/more polypeptide chains)
  • Carbohydrates
  • Lipids
  • Nucleic acids
• Types of antigens

  • Self-antigens
  • Non self-antigens (foreign)
• Self-antigens


  Present on the surface of your own cells (e.g. Red blood cells A, B, AB, O)

  
• Non self-antigens (foreign)


  Foreign molecule present on cells, circulate freely in body fluids (e.g. Bacteria)
• Allergens

  • Antigens that cause allergic reactions
  • Overreactive immune response (such as inflammatory response)
  • Provokes hypersensitivity to a harmful substance
• Responding to antigens

  • B lymphocyte recognition (membrane-bound antibodies, secreted antibodies)
  • T lymphocyte recognition (receptors on T lymphocytes, no secreted antibodies)

  
• Self-tolerance


  Inability to respond to self-antigens
• Autoimmune diseases

  Absence of self-tolerance
• MHC markers

  • MHC-I (present on all nucleated cells except red blood cells, presents antigens from virus infected cells, cancer cells, cytotoxic T cells)
  • MHC-II (only found in certain immune cells, presents antigens from phagocytosed and foreign cells)

  
• Pathogens

  • Primary pathogens (can cause disease at any time)
  • Opportunistic pathogens (only cause disease on weakened host defence)
• Cellular pathogens

  • Bacterial (extracellular, intracellular)
  • Fungi
• Bacterial pathogens

  • Some can live and reproduce only on the external surfaces of human cells or in body fluids
  • Some can live and reproduce either inside or outside the cells of their host
  • Some can only survive and reproduce within host cells
• Bacterial toxins

  • Exotoxins (secreted toxins)
  • Endotoxins (parts of outer membrane released when cell dies)
• Fungi


  • Unicellular eukaryotic cell
  • Reproduce by budding
  • Spreads by releasing spores
  • Invade the host with thread-like hyphae
  • Secrete digestive enzymes that break down host tissues and then absorb them
• Types of fungal infections

  • Subcutaneous (soil or plant material, direct implantation of spores into skin via a puncture wound)
  • Cutaneous (infected person or spores, fungi break down keratin protein in hair, skin, nails)
  • Systemic (occurs inside the body, usually starting from lungs, inhalation of spores)

  
• Non-cellular pathogens


  • Viruses (nucleic acid core, capsid, envelope, unable to function outside of host cell)
  • Prions (composed of only protein, abnormal folding to make infectious, seeks and converts normal proteins into prions, causes brain disease)
• Replication of viruses

  1. Virus attaches to specific host cell

  2. Virus injects nucleic acid material into host cell

  3. Uses host organelles and energy to transcribe more viral genes, destroying host cell

  4. Viral particles bud off the cell and infect other cells

  
• Retroviruses

  Contains RNA instead of DNA, contain enzyme reverse transcriptase (RNA TO DNA)
• Transmission of prions

  • Infection (consumption of infected meat, exposed to contaminated surgical instruments)
  • Hereditary (autosomal dominant trait)
• Adaptive immune response is unable to identify and respond to prions due to their similar normal proteins
• Differences between bacteria and viruses

  • Bacteria (cellular pathogens, cell membrane, circular DNA, ribosomes)
  • Viruses (non-cellular pathogens, protein coat, DNA or RNA, no ribosomes)
• Innate (non-specific) immunity

  Individuals are born with it, same response to every pathogen, pre-existing defence, no immunological memory, quicker

  
• Adaptive (specific) immunity

  Antigen-specific immune response, different response for each pathogen, immunological memory acquired, occurs when innate immune defences are evaded, takes longer

  
• Cell-mediated and humoral responses

  • Cell-mediated immunity (immune response mediated by immune cells, kill)
  • Humoral immunity (immune response mediated by soluble molecules, disable)

  
• Cells of the immune system

  • All are white blood cells, derived from stem cells in the bone marrow, subgroup is lymphocytes (B lymphocytes and T lymphocytes)

  
• Lines of defence

  • First line (non-specific, physical, chemical and microbiological barriers, e.g. intact skin and mucous membrane)
  • Second line (non-specific, immune cells and soluble proteins of the innate immune system, attack pathogens that gain entry, inflammation)
  • Third line (specific, immune cells of the adaptive immune system, various actions of T cells and B cells, antibodies tailored to each invading pathogen)

  
• Physical barriers in animals

  • Epithelial cells (skin, respiratory, gastrointestinal and urogenital tract)
  • Intact skin
  • Mucus membranes (respiratory tract, traps pathogens)
• Physical barriers in plants

  • Cutin and waxes
  • Bark
  • Stomata
  • Leaves
  • Cell wall
  • Thorns and spikes
• Chemical barriers in animals

  • Secreted fatty and lactic acids (tears, sweat, saliva)
  • Secreted stomach acid and digestive enzymes
  • Fluids containing surfactant (lung fluid)
• Chemical barriers in plants

  • Saponin (wheat)
  • Caffeine (coffee, tea, cocoa plants)
  • Tannins (tea, grapes)
  • Citronella (lemongrass)
  • Defensins
• Thorns and spikes

  • Modified leaves and branches that act as a physical defence to grazing animals
• Chemical barriers in animals


  Secretion of substances (chemicals or enzymes) that are present to kill pathogens
• Chemical barriers in animals

  • Secreted fatty and lactic acids contain lysosomes that break down pathogens
  • Secreted stomach acid and digestive enzymes kill pathogens during digestion
  • Fluids containing surfactant that coat pathogens to make it easier for macrophages to kill
• Chemical barriers in plants

  Secretion of substances (chemicals or enzymes) that are present to kill pathogens
• Chemical barriers in plants

  • Saponin in wheat disrupt cell membranes of fungi
  • Caffeine in coffee, tea, cocoa plants is toxic to insects and fungi
  • Tannins in tea and grapes are toxic to insects
  • Citronella in lemongrass repels insects
  • Defensins in barley and wheat are toxic to microbes
  • Chitinases in barley, tomato, banana disrupt cell membranes of fungi
• Microbacterial barriers in animals

  Presence of flora (non-pathogenic bacteria) create a physical barrier against pathogens
• Microbacterial barriers in animals

  • Flora prevents growth and colonisation of other bacteria
  • Flora outcompetes pathogen for resources and space
  • Flora produces chemicals that reduce pH of microenvironment