Immunology

• Pathogens


  • Disease-causing microorganisms
  • Bacteria
  • Viruses
  • Fungi
• Disease occurs when an infection leads to recognisable symptoms in the host
• When a pathogen is transferred from one individual to another, it is known as transmission
• Viruses


  • They are acellular and non-living
  • They have no nucleus, organelles, cell-surface membrane, or cytoplasm
  • A typical virus particle contains genetic material (DNA or RNA), a capsid (consisting of protein), and attachment proteins on the outside
• Replication of viruses


  1. Virus particles attach to specific host cells using attachment proteins/glycoproteins

  2. Most viruses inject their nucleic acid (RNA or DNA) into host cells

  3. Genetic material is used to code for more virus particles produced using host cell organelles

  4. This involves producing copies of viral nucleic acids and proteins to form complete viruses often released by lysis of the cell
• Phagocytosis ('cell eating')


  1. Engulfment and destruction of microorganisms by phagocytic white blood cells

  2. Phagocyte extends around the pathogen, engulfs it forming a phagosome or phagocytic vesicle

  3. A lysosome containing lysozymes fuses with the phagosome, enzymes hydrolyse the pathogen

  4. Soluble digested products are absorbed, indigestible material is removed

  5. Phagocytes can act as 'antigen-presenting cells'
• Phagocytes destroy microorganisms that enter the blood and other tissues helping to reduce their spread to other parts of the body
• Specific Defence Mechanisms


  • Immune system and a specific response against a pathogen, abnormal cell, or toxin
  • Responses often provide long-lasting immunity and involve B lymphocytes and T lymphocytes
• Antigens


  • Proteins or glycoproteins that appear 'foreign' to the individual organism exposed to them
  • Stimulate the production of antibodies by B lymphocytes
  • May be present on the surface of a pathogen, cell-surface membrane of other organisms of the same species, abnormal body cells, or as a toxin
• The Humoral Response - B cells

  1. Body has different types of B lymphocytes, each capable of producing a specific antibody

  2. B lymphocytes secrete small amounts of their specific antibody onto their cell surface membrane

  3. Specific antigen may attach to the complementary antibody on B lymphocytes

  4. B-cells are stimulated to divide by mitosis, known as Clonal Selection

  5. Helper T cells activate the B-cells to divide

  6. Mitosis results in a large population of identical plasma cells, a clone of plasma cells

  7. Plasma cells produce the same specific antibody and secrete it into the blood plasma

  8. Some B cells develop into memory B cells
• Antibodies secreted by plasma B cells bind specifically to antigens forming an antibody-antigen complex, leading to the destruction of the antigen/pathogen
• The primary response is relatively slow, requiring up to 72 hours to produce a significant concentration of antibody
• During the primary response time, microorganisms reproduce
• Plasma B cells binding to antigens forming an antibody-antigen complex


  Stimulates processes leading to the destruction of the antigen/pathogen
• Memory B Cells and Immunity

  1. Primary response is slow, taking up to 72 hours to produce a significant concentration of antibody

  2. Memory B-cells divide and develop into plasma cells upon subsequent encounter with the same antigen (pathogen), leading to a secondary response with quicker and higher antibody secretion providing immunity
• Antigenic variation


  Some microorganisms like the influenza virus have a high mutation rate leading to antigenic variation, making immunity to one strain ineffective against new forms
• Antibodies


  Proteins produced by B lymphocytes that destroy or neutralise antigens and pathogens
• Locations of antibodies


  • Blood plasma
  • Tissue fluid
  • Breast milk
• Structure of an antibody molecule

  • Consists of four polypeptide chains - 2 heavy and 2 light chains joined by disulfide bonds
  • Each chain has a constant region and a variable region
• Variable regions of antibody chains

  Form two antigen-binding sites with a specific tertiary structure complementary to the antigen molecule they attach to
• Antibodies do not directly destroy antigens/pathogens

  Stimulate processes like agglutination and phagocytosis that lead to destruction
• Agglutination


  Refers to the clumping together of cells possessing the antigen against which specific antibodies react, aiding in destruction e.g. by phagocytosis
• Stimulation of phagocytosis


  One type of antibody attaches to the antigen on the pathogen's surface, identifying it for destruction by phagocytic white blood cells
• The Cellular Response - T cells

  1. T cells have receptors that bind to specific antigens, each type of T cell responds to a different antigen

  2. Helper T cells (TH cells) bind to antigens, divide to form a clone, and activate cytotoxic T cells (TC cells) to destroy pathogens
• Cloned TH cells


  1. Activate cytotoxic T cells (TC cells) - which attach to the specific antigen on the pathogen/‘foreign cell’ and secretes chemicals (e.g. a protein called perforin) to destroy it

  2. Develop into more helper T cells (TH cells) which stimulate B lymphocytes to divide into plasma cells and secrete antibodies

  3. Develop into memory T cells - which remain in the blood after the infection has cleared and produce a quicker response (secondary response) if a future infection occurs with the same antigen/pathogen
• Types of Immunity


  • Passive immunity
  • Active immunity
• Passive immunity is where an individual receives pre-formed antibodies from an outside source. The individual is not exposed to the antigens and does not form antibodies or memory cells. Although antibodies will be immediately present, this only provides short-term immunity as the antibodies are not produced by the individual and are not replaced once they are broken down
• Natural Passive Immunity


  • The antibodies are obtained across the placenta and through breast milk. This provides short-term protection, as the body is not stimulated into producing its own antibodies and memory cells
• Artificial Passive Immunity


  • The pre-formed specific antibodies are injected usually following exposure to particularly infectious pathogens or toxins e.g. the rabies virus, snake venom (toxin). Again, this provides short-term protection
• Active immunity is where an individual exposed to the antigen produces antibodies and memory cells. Although it takes time to produce these antibodies, the immunity is usually long-term as the immune system has produced its own antibodies and memory cells
• Natural Active Immunity


  • This results from an individual becoming infected e.g. chicken pox, and is exposed to the specific antigen. Long-term immunity develops as memory cells are formed
• Artificial Active Immunity


  • This results from an individual being vaccinated (immunisation) and the immune system produces its own antibodies and memory cells providing long-term immunity
• Vaccination


  A vaccine contains antigens from a specific pathogen. The antigens in the vaccine may be present in the form of the dead pathogen, a weakened strain of the pathogen or just antigens (removed from the pathogen). Injection of this vaccine stimulates an immune response with the production of plasma cells, which release specific antibodies, memory B cells and memory T cells. These memory cells provide long-term immunity. Sometimes a ‘booster’ injection of the same antigen is given at a later date, to ensure a more effective response (secondary response) due to exposure to more antigen producing long-term - often life-long – immunity, due to the memory cells formed
• ‘Herd Immunity’ - The higher the percentage of the population vaccinated against a particular antigen/pathogen, the less the risk of transmission. This is because the probability of an infected person encountering a person without immunity will be low. A very important factor in people getting vaccinated is the perceived safety of the vaccine and its effectiveness in providing protection
• HIV and AIDS - Acquired immune deficiency (AIDS) syndrome is caused by infection with the human immunodeficiency virus (HIV)
• HIV is a retrovirus containing RNA and the enzyme reverse transcriptase which produces DNA in the host cell using RNA as a template. The RNA and enzyme are surrounded by a capsid (protein coat). The capsid is surrounded by a lipid envelope which contains glycoprotein ‘spikes’. Specific glycoproteins on the surface of the virus enable it to attach to its host cell, helper T cells (TH cells)
• Transmission of HIV - HIV is transmitted only by the introduction of blood, semen or vaginal secretions from an infected individual into the bloodstream of another individual. This can occur by: Sexual transmission, Blood products/transfusions of infected blood, Sharing of needles e.g. drug users, Mother to baby (via placenta, during child birth or via breast milk)
• HIV replication


  The host cell for HIV is the helper T cell. The virus attaches using their glycoprotein ‘spikes’ which are complementary to specific protein receptor sites on the cell-surface membrane of the helper T cells. The lipid envelope fuses with the cell-surface membrane and viral RNA and the enzyme reverse transcriptase are released into
• The helper T cell is the cell for HIV
• Virus attachment to helper T cell

  The virus attaches using their glycoprotein ‘spikes’ which are complementary to specific protein receptor sites on the cell-surface membrane of the helper T cells