Immunity

• Most microorganisms are harmless or even beneficial for example, the many types of bacteria and fungi involved in decay and decomposition are essential for life on Earth
• A small percentage of microorganisms, including some bacteria, fungi, protoctists and viruses (although technically viruses are not classified as living organisms) can cause disease or be pathogenic
• If a pathogen gains entry to the body it could cause significant harm or even death
• Body's first line of defence against pathogens


  • Try to prevent entry
• Non-specific defence mechanisms


  Not specific to individual types of pathogens. Phagocytosis is an example
• Specific immune response


  Distinguishes between individual pathogens and the response is tailored to the pathogen involved. Takes longer to work but tends to provide long term immunity
• Specific immune responses


  • Involve lymphocytes, a special type of white blood cell
• Lymphocyte development


  1. Mature in bone marrow

  2. Migrate to thymus gland
• Antigens are molecules that stimulate an immune response, while antibodies bind to antigens on pathogens or abnormal cells.
• The immune system is the body's defense against foreign substances.
• Phagocytic cells engulf foreign particles through endocytosis and destroy them with enzymes.
• Barriers preventing the entry of pathogens in humans

  • Outer protective covering (skin)
  • Lysozyme in body secretions
  • Mucus-covered respiratory tract
  • Hydrochloric acid in stomach
• Skin


  Provides a tough physical barrier that most pathogens cannot penetrate
• Skin ceases to be an effective barrier

  If it is punctured (e.g. wound or cut) or not in its normal healthy condition
• Lysozyme


  Enzyme contained in many body secretions including tears that can digest (hydrolyse) bacterial cell walls
• Tears


  Can wash away debris and pathogens from the front of the eye
• Respiratory tract


  • Covered in mucus that traps pathogens and prevents them penetrating the underlying membranes
  • Cilia (tiny hairs) that line the respiratory tract sweep the mucus-trapped pathogens up the trachea
• Hydrochloric acid in stomach

  Kills most pathogens that are in the food we eat or the liquids we drink by providing a very low pH that denatures the cells of the pathogens
• Despite these (and other) barriers to pathogen entry, many do invade our body
• The next line of defence is phagocytosis
• Phagocytosis


  Non-specific but rapid process carried out by white blood cells (phagocytes) to engulf pathogens and cell debris
• Phagocytosis


  1. Phagocyte moves towards pathogen

  2. Phagocyte membrane encloses pathogen

  3. Phagosome forms around pathogen

  4. Lysosomes fuse with phagosome

  5. Hydrolytic enzymes digest pathogen

  6. Digested products absorbed into phagocyte
• Phagocytes


  • Include polymorphs (most common, first to arrive) and macrophages (develop from monocytes, larger and longer-lived than polymorphs)
  • Able to squeeze through capillary walls and engulf pathogens and cell debris at site of infection
• Inflammation following infection


  Capillaries become leaky, allowing fluid to seep into surrounding area
• Inflamed areas appear swollen due to increased blood flow
• Inflamed areas also become hot
• Phagocytes contain digestive enzymes
• Specific immune response


  Associated with lymphocyte white blood cells
• Specific immune responses


  Triggered by the body being able to recognise "foreign' cells, linked to the concept of self and non-self tissue
• Self


  Cells recognised by the body
• Non-self


  Foreign cells not recognised by the body, will produce an immune response
• Antigens


  Specific molecules or clusters of molecules that form part of the cell surface membrane and are recognised as foreign
• Examples of antigens


  • Protein
  • Polysaccharides
  • Glycoprotein
  • Glycolipid
• Different pathogens have different antigens, consequently the immune response is specific to these antigens
• Lymphocyte receptor


  Complementary in shape to the antigen, fit together like substrate and enzyme in the lock and key model of enzyme action
• Antigens can be defined as chemicals/molecules capable of producing a specific immune response
• How lymphocytes know what is self and non-self

  1. Many million different types of lymphocytes, each having receptors with a complementary shape to a potential antigen

  2. In the foetus, these lymphocytes frequently make contact with other foetal (self) cells

  3. Lymphocytes that are complementary in shape with foetal cells are switched off

  4. By the time the baby is born, the functional lymphocytes that remain are those that are not complementary to self cells

  5. However, because there are many million functional lymphocytes remaining there are only a few of each type
• Types of lymphocyte


  • B-lymphocytes (B and lymphocyte-cells)
  • T-lymphocytes
• 1. B-lymphocytes

  • Formed from stem cells in the bone marrow
  • Mature in bone
  • Produce antibodies which respond to antigens found in body fluids or example, blood and sus fluid
  • Respond to bacterial or viral infection
• 1. T-lymphocytes

  • Formed from stem cells in the bone marrow
  • Mature in thymus gland
  • Respond to antigens matched to body cells
  • Respond usually to body cells affected by infection