Unit 4 Immunity

• Virus: composed of genetic material (DNA or RNA) inside a protein coat (capsid)

  Protozoa: single-celled eukaryote that can cause disease

  Fungus: contain long branching filaments called hyphae

  Bacterium: a prokaryote which often produces toxins

  Prion: abnormally folded proteins
• pathogen: an agent that causes disease

  antigen: any molecule that may trigger an immune response
• non-self antigen a molecule from outside the body that is recognised by the immune system and initiates an immune response. Also known as a foreign antigen
• major histocompatibility complex (MHC) markers a group of proteins present on the surface of all self-cells that enables the immune system to distinguish it from non-self material.

  • major histocompatibility complex class I (MHC Class I) markers expressed on all nucleated cells in the body. These mark cells as ‘self’ so that the immune system doesn’t attack them
  • major histocompatibility complex class II (MHC Class II) markers expressed on antigen - presenting cells, which interact with T helper cells in the process of antigen-presentation
• • Cellular pathogens have a cellular structure and are living organisms.
  • Non-cellular pathogens do not have a cellular structure and are non-living.
• Whenever pathogens affect the normal functioning of our cells, we say that a disease is occuring. For example, some pathogens may produce toxins which cause the lysis of cells, some may inhibit protein or nucleic acid synthesis, and others may affect cellular respiration, preventing cells from producing energy.

  lysis the disintegration or rupturing of a cell
• Extracellular threats are threats found outside of a cell that can interfere with its functioning, such as a bacteria. In contrast, intracellular threats are threats found within a cell that can also interfere with its functioning.
• Bacteria (cellular)

  Unicellular prokaryotes that can infect almost any part of the body. Bacteria can cause disease through the production of toxins and enzymes which either affect the functioning of cells or cause their death.

  Bacteria typically reproduce asexually via binary fission.
• Fungi (cellular)

  Eukaryotic organisms that include yeasts and moulds and contain long, branching filaments called hyphae.

  Fungi reproduce both through aesexual reproduction and sexual reproduction via spore formation.
• Worms (cellular)

  Multicellular invertebrate parasites whose development include egg, larval, and adult stages. Can vary in length, with the longest worms being over 55 m in length.

  Worms reproduce sexually via a complex life cycle.
• Protozoa (cellular)

  Single-celled eukaryotes that can be free-living or parasitic. Protozoa have many different mechanisms of action – for example, some can inhibit nucleic acid synthesis, protein synthesis, and various stages of cellular respiration.

  Protozoa reproduce both through asexual reproduction and sexual reproduction.
• Viruses (non-cellular)


  An infectious agent composed of genetic material (DNA or RNA) inside a protein coat (capsid)
• Viral replication


  1. Viruses insert their genetic material into a host's cell

  2. Viruses use the cell's machinery to replicate
• Viruses


  • They are not able to independently reproduce
• Viruses


  • Can cause disease through the lysis of cells during viral replication
  • Can cause the formation of cancer by affecting gene expression
  • Can cause over-stimulation of the immune system leading to organ damage
• Prions (non-cellular)

  Abnormally folded proteins that have the ability to induce normal proteins nearby to become misfolded. They only occur in mammals and affect only the brain and other neural structures. They are currently the only known infectious agents that don’t contain nucleic acids.

  Prions do not reproduce - instead, they induce misfolding in nearby proteins, thereby ‘spreading’ throughout a tissue.
• Innate immune system


  A component of the immune system that is composed of generalised and non-specific defences and/or responses to pathogens. Also known as the non-specific immune system.
• First line of defence

  • A component of the innate immune system characterised by the presence of physical, chemical, and microbiological barriers to keep pathogens out of the host organism
• Second line of defence

  • A component of the innate immune system characterised by the non-specific response to injury and/or pathogens by a variety of cells and molecules
• Non-specific


  Describes a component of the immune system that responds the same way to all pathogens
• Physical barrier


  • A component of the first line of defence that features solid or fluid obstacles that block pathogen entry such as skin or mucus
• Chemical barrier


  • A component of the first line of defence that features the use of enzymes, toxins, and acids to protect against pathogen invasion
• Cuticle


  A waxy protective film covering the surface of a plant leaf
• Gall


  An abnormal outgrowth of tissue in plants designed to limit the spread of an invading pathogen
• Trichomes


  Small hairs on the surface of plants used to deter pathogens and/or insects
• Stoma (pl. stomata)


  A small pore on the leaf's surface that opens and closes to regulate gas exchange
• Microbiological barrier


  • A component of the first line of defence in which the presence of normal flora limits the growth of pathogenic bacteria. Also known as microbiota barrier
• Phagocytes


  Cells that engage in phagocytosis, a process in which they consume and destroy foreign or dead material present in the body by engulfing it through the process of endocytosis
• Phagocytes


  • Neutrophils
  • Macrophages
  • Dendritic cells
• Antigen-presenting cells


  Macrophages and dendritic cells that not only consume and destroy foreign material, but also present antigens from consumed material on their surface
• Antigen-presenting cells


  • Express MHC Class II to present the consumed antigens on their surface
  • Use their MHC Class II markers with the presented antigen to interact with the adaptive immune system
• Cytokines


  Important cell signalling molecules which help protect against pathogens and can help guide immune cells to the site of infection or injury, allowing them to function optimally
• Natural killer (NK) cells

  Large granulated cells which target both abnormal and virally infected cells
• Natural killer (NK) cells

  • Have a killer inhibitory receptor that examines the surface of cells for MHC Class I markers
  • Have a killer activation receptor that binds to certain molecules which appear on cells undergoing cellular stress (e.g. infected or cancerous cells)
• MHC Class I markers are absent


  Due to the presence of a viral infection, which can either destroy or suppress the production of MHC Class I markers
• MHC Class I markers are absent


  Due to the gene expression of MHC Class I markers being affected in cancer cells
• Killer inhibitory receptor detects a sufficient number of MHC Class I markers


  It overrides the killer activation signal, preventing cell death
• Killer activation receptor is activated and the killer inhibitory receptor is unable to bind to a sufficient number of MHC Class I markers

  Cell death is initiated in infected or abnormal cells with missing MHC Class I markers
• Mast cells


  Reside in connective tissues throughout the body. When they detect injury to surrounding cells or are stimulated by antigens or allergens, they become activated and degranulate, releasing histamine. Histamine has a number of effects on the body and is particularly important in the inflammatory response.
• Eosinophils


  Large granulated cells containing various toxic chemical mediators such as DNases, RNases, and proteases, which help destroy invading pathogens. They typically target pathogens which are too large to be phagocytosed by degranulating on contact with them and releasing the chemical mediators contained within their granules.