immunity

• Defence mechanisms against pathogens

  • Preventing the entry of pathogens by physical and chemical defences such as skin, mucous membranes, tears, saliva
  • Inflammation of the region invaded by the pathogen
  • Recognising 'foreign' cells and targeting pathogenic cells
• Inflammation

  Swelling and heating of the region invaded by the pathogen, a non-specific inflammatory response
• Recognising 'foreign' cells

  Targeting any pathogenic cells, a specific immune response
• Specific molecules on the surface of cells


  Enable the body to identify 'foreign' cells
• Surface molecules

  Usually proteins, part of the phospholipid bilayer, such as glycoproteins
• Glycolipids


  Can also act as markers for cell recognition
• Surface proteins

  Enable the body to recognise pathogenic cells, abnormal body cells, toxins, cells from other individuals of the same species
• Antigens


  Surface molecules used by the body to identify cells, viral particles, and toxins
• Phagocytes have surface proteins that act as receptors and bind to the proteins (antigens) on the surface of pathogens
• Antigens

  Markers found on cell surface membranes, bacterial cell walls, or the surfaces of viruses that allow cell-to-cell recognition
• Every cell in the human body has markers that identify it
• Microorganisms (both pathogenic and non-pathogenic), such as bacteria and viruses, also have their own unique markers
• Self antigens


  Antigens produced by the organism's own body cells that the immune system does not recognise as foreign antigens
• Self antigens do not stimulate an immune response
• Non-self antigens


  Antigens not produced by the organism’s own body cells that the immune system recognises as being foreign
• Non-self antigens stimulate an immune response
• Some glycolipids and glycoproteins on the outer surface of cell surface membranes act as antigens
• Some pathogens exhibit antigen variability

  The antigens present on the surface of pathogens change frequently due to genetic mutations

  This poses a problem for the immune system of many mammal hosts as lymphocytes and memory cells produce a specific immune response

  The surface receptors on lymphocytes and memory cells are complementary in shape to only one antigen

  When the antigen on a pathogen changes, the lymphocytes and memory cells can no longer bind, resulting in no secondary immune response

  The host gets infected and suffers from the disease
• The cold virus and flu virus are common pathogens that exhibit antigen variability
• Phagocytes


  White blood cells produced continuously in the bone marrow, responsible for removing dead cells and invasive microorganisms, carry out a non-specific immune response
• Phagocytosis


  The process of recognising and engulfing a pathogen
• Main types of phagocyte

  • Neutrophils
  • Macrophages
• Neutrophils


  • Travel throughout the body, leave the blood by squeezing through capillary walls to 'patrol' body tissues, released in large numbers during infections, short-lived cells
• Lysosomes


  Membrane organelles containing digestive enzymes called lysozymes, digest unwanted material present in cells
• Phagosome

  The phagocytic vacuole formed around a pathogen once it has been engulfed by a neutrophil
• Pus is a sign of dead neutrophils
• Macrophages


  Move into organs including the lungs, liver, spleen, kidney, and lymph nodes, play a very important role in initiating an immune response, develop from monocytes once they leave the blood to settle in various organs
• Mode of action for Macrophages

  Carry out phagocytosis, cut pathogens up to display antigens on their surface, antigens can be recognised by lymphocytes
• Neutrophils

  Chemicals released by pathogens and chemicals by body cells under attack (histamine) attracts neutrophils to the site of pathogens
• Neutrophils

  • Some may have antibodies- another trigger to stimulate neutrophils to attack pathogens
• mode of neutrophils


  
  

  1. chemicals of pathogens attract neutrophils

  2. Neutrophils attach to pathogens (may have antibodies- another trigger to simulate neutrophils to attack)

  3. Phagocytic vacuole formed around pathogen when engulfed by neutrophils is called phagosome, this part of process is endocytosis

  4. Lysosomes release lysozymes to digest the pathogen

  5. Lysozyme fuse with the membrane of the phagosome and releases lysozymes and destroys pathogen
• Neutrophils die after killing and digesting pathogens
• Lymphocytes


  Type of white blood cell
• Specific immune response

  Immune response that targets specific pathogens
• Lymphocytes


  • Smaller than phagocytes
  • Large nucleus that fills most of the cell
  • Produced in the bone marrow before birth
• Types of lymphocytes

  • T-lymphocytes
  • B-lymphocytes
• Maturation of T-lymphocytes

  1. Immature T-lymphocytes leave the bone marrow to mature in the thymus

  2. Mature T-lymphocytes have specific cell surface receptors called T cell receptors

  3. These receptors have a similar structure to antibodies and are each specific to one antigen

  4. The maturation of T-lymphocytes results in some becoming helper T cells and others becoming killer T cells

  5. T-lymphocytes are activated when they encounter (and bind to) their specific antigen that is being presented by one of the host’s cells
• Antigen-presenting


  Host cell presenting the antigen on its cell surface membrane
• Activation of T-lymphocytes

  1. Activated T-lymphocytes divide by mitosis to increase in number

  2. These T-lymphocytes differentiate into two main types of T cell: helper T cells and cytotoxic T cells
• Helper T cells


  Type of T cell that helps activate other immune cells