Immunity

• What are most microorganisms?

  Most microorganisms are either harmless or beneficial, but a small percentage of bacteria, fungi protoctists and viruses are pathogenic.
• What does the term immunity refer to?


  The term immunity refers to the action of the body's immune system and how it deals with infectious diseases (those caused by invading pathogens).
• For a microorganism to be pathogenic it must...

  1. Enter the host

  2. Colonise tissues

  3. Evade the host's defences

  4. Cause damage to the host tissues
• What happens if a pathogen enters the body?

  If a pathogen enters the body, and colonises its tissues, an infection occurs.
• When does disease occur?

  Disease occurs when the infection causes recognisable symptoms.
• What may the disease damage?

  The disease may damage the host tissues directly (viruses break down cells), or through the production of toxins (bacteria release many toxins).
• What is the body's first line of defence?

  Prevent entry of pathogens into the body.
• What defence mechanisms are available if pathogens enter the body?

  - Non-specific

  => These are not specific to the individual pathogen (e.g. phagocytosis)

  - Specific Immune response.

  => This does distinguish between pathogens and involves lymphocytes (a WBC).
• What is the response tailored to + what is the length of immunity provided?

  The response is tailored to the pathogen involved, and although it takes longer to work, it tends to provide long term immunity.
• What are 4 natural barriers to pathogen entry?

  - Skin

  - Tears

  - Epithelial linings covered in mucus

  - Hydrochloric acid
• Give the role of skin (an outer protective covering) in preventing pathogen entry. (4)

  - This acts as a tough physical barrier to pathogens.

  
  

  - It is waterproof and produces sebum - an oily substance with antiseptic properties.

  
  

  - It also produces sweat - a mixture of chemicals that includes lactic acid and lysozyme.

  
  

  - Once punctured (wound or cut) it ceases to be an effective barrier to pathogens
• Give the role of tears in preventing pathogen entry. (3)


  - These protect the eyes, keeping them moist at all times.

  
  

  - Blinking constantly refreshes the tears covering the eye removing any dust that may contain bacteria

  
  

  - Tears contain the enzyme lysozyme - this digests (hydrolyses) the cell walls of bacterial cells resulting in their death.
• Give the role of epithelial linings, covered in mucus, in preventing pathogen entry. (4)

  - Mucus acts as an impenetrable barrier to bacteria and other pathogens in various body systems including the respiratory, digestive and reproductive systems.

  
  

  - It prevents them penetrating underlying membranes

  
  

  - It also contains lysozyme.

  
  

  - Cilia (tiny hairs in the respiratory tract) sweep the mucus and trapped pathogens back up the trachea
• Give the role of hydrochloric acid in preventing pathogen entry. (3)


  - Kills most pathogens in the food or drink that we consume

  
  

  - The low pH denatures the enzymes of the pathogen allowing the body to protect

  
  

  - e.g. HCl in the gastric juice in the stomach
• What is the next line of defence if these non-specific barriers fail?

  Phagocytosis.
• What is phagocytosis?


  Phagocytosis is rapid and non-specific, and is carried out by WBCs called phagocytes.
• What does the inflammatory response of phagocytes following infection involve? (5)

  - The capillaries in the affected area becoming leaky + allowing plasma to seep into the affected areas.

  
  

  - Phagocytic white blood cells can also squeeze through the capillary walls and accumulate in the affected area.

  
  

  - Inflamed areas become swollen with pus containing phagocytes, dead pathogens and cell debris

  
  

  - Increased blood flow to the inflamed area (it appears red)

  
  

  - The inflamed area becomes hot - his denatures the enzymes in the pathogen and helps to reduce the infection.

  
• What two type of cells can phagocytes be categorised as?

  - Polymorphs: most common phagocyte + first to arrive and

  
  

  - Macrophages: larger and longer lived (develop from monocytes)
• Explain the process of phagocytosis. (5)

  - Chemicals produced by the pathogen, attract the phagocyte so it moves towards it

  
  

  - The phagocyte membrane invaginates to enclose the pathogen, and engulfs it, forming a vesicle (phagosome) around the pathogen

  
  

  - Lysosomes move towards the phagosome and fuse with it

  
  

  - Hydrolytic enzymes in the lysosome are released into the phagosome, which hydrolyse the pathogen within the phagosome.

  
  

  - Soluble digested products are absorbed into the cytoplasm of the phagocyte.
• What is the specific immune response associated with + what do they react to? (2)

  - The specific immune response is associated with lymphocytes, another type of WBC.

  
  

  - They react to antigens present of the surface of pathogens which they recognise as foreign or "non-self"
• What is an antigen?


  An antigen is a molecule found on the surface of living cells.
• What types of molecule can antigens be?


  They are usuallyproteinsbut may also be nucleic acids, polysaccharides, glycoprotein, glycolipids or even inorganic molecules.
• Comment on the presence of antigens on living cells?

  All living cells have antigens and each organism, due to its unique genetic make-up, will produce different antigens.
• Explain the concept of 'non-self' antigens.

  The immune system is able to recognise those antigens that belong to their body - self antigens + those that are foreign and don't belong to the organism - non-self antigens.
• What is the immune system's response to antigens due to?

  This specific response is due to the lymphocytes having specific protein receptors on their cell surface membrane that are complementary in shape to the antigen.
• Comment on the appearance of lymphocytes in comparison to their biochemical structure.

  Lymphocytes all look the same but differ biochemically, as they carry different receptors that are specific to the different antigens that be encountered in foreign cells.

  
• When are lymphocytes first produced in the life cycle of a human + what does each carry? (2)


  - During early development, many millions of different lymphocytes are produced from the stem cells in the bone marrow.

  
  

  - Each carries a specific membrane receptor that allows it to response to a different non-self antigen, if it should be encountered in the future.
• What do lymphocytes not respond to?

  Lymphocytes do not respond to chemical on the surface of an individual's own cells (self markers).
• Why do lymphocytes not respond to chemical on the surface of an individual's own cells?

  This is because in the foetus, the lymphocytes frequently make contact with other foetal cells (self cells), and the lymphocytes that are complementary in shape to the foetal cell markers are "switched off", so by the time the baby is born the functional lymphocytes that remain are not complementary in shape to any of the baby's markers (self).
• Why is the specific immune response relatively slow?

  There are so many different functional lymphocytes (millions) in an organism, but there are only a few of each type.
• What are the 2 cell types responsible for the immune response?

  B and T lymphocytes

  
• What is the general function of B + T lymphocytes?

  These cells work to specifically target antigens, but respond in a different ways to deal with the infection.
• What does the activation of a lymphocyte involve + what are there only a few of available in the blood? (2)

  - Activation of a lymphocyte involves it coming into contact with a non-self antigen that its receptor recognises.

  
  

  - Remember there are only a few of the correct B or T lymphocytes available in the blood.
• Briefly describe the role of B-lymphocytes in the specific immune response. (3)


  - A B lymphocyte will recognise the antigen on the pathogen itself, and become sensitised.

  
  

  - When its receptors are sensitised, it will activate the gene responsible for the production of antibodies - the antibody mediated response.

  
  

  - Eventually the antibodies will be secreted into the blood stream, and target the antigens on the pathogen.
• Briefly describe the role of T-lymphocytes in the specific immune response. (2)


  - If a T lymphocyte is stimulated a number of different types of T cells are produced, each having a different job.

  
  

  - Ultimately the T cells will directly destroy the cell - the cell mediated response
• How do B + T lymphocytes divide?

  In both B and T lymphocytes, the sensitised lymphocytes then divide by mitosis (cloned) and differentiate into a variety of cells.
• How long does the delay in the specific immune response take + what happens during this time period?

  This delay can take time (approx. 4 days) between the contact of the antigen and the cloning of the required lymphocyte, and it is during this time delay that the person suffers from symptoms of the disease caused by the pathogen.
• What is anti-body mediated immunity?

  This type of immunity specifically targets microorganisms that are found in bodily fluid, rather than in body cells, and involves the production of antibodies.
• Describe the shape of antibodies in terms of aiding their function.

  Antibodies are specifically shaped globular protein molecules each one responsible for destroying a specific pathogen - i.e. they have a shape complementary to the antigen on the surface of the pathogen.
• Describe the primary immune response. (3)

  - The B lymphocytes exist as millions of different types each having a unique cell surface receptor.

  
  

  - When a specific antigen present on the surface of a pathogen, sensitises its specific B lymphocyte, the B lymphocyte will become cloned, and produce plasma cells and memory cells.

  
  

  - This initial response of the body when encountering an antigen for the first time is known as the primary immune response.