infections and immunity


Published on

this is a series of lectures on microbiology useful for undergraduate medical and paramedical students

Published in: Health & Medicine, Technology
  • Be the first to comment

infections and immunity

  1. 1. 1 of 41 © Boardworks Ltd 2006
  2. 2. • Pathogens • Microorganisms that cause diseases • Commensals • Those that live in harmony with the host without causing any damage to it 2 of 41 © Boardworks Ltd 2006
  3. 3. INFECTION • Lodging and multiplication of pathogens in or on tissues of a host 3 of 41 © Boardworks Ltd 2006
  4. 4. Primary infection • Infection that develops in an otherwise healthy host 4 of 41 © Boardworks Ltd 2006
  5. 5. Re - infection • Infection by the same organism again in the same host 5 of 41 © Boardworks Ltd 2006
  6. 6. Secondary infection • Infection in the host affected by primary infection by a new organism 6 of 41 © Boardworks Ltd 2006
  7. 7. Disease-causing organisms Organisms that cause disease are called pathogens. What are the four major types of pathogen? bacteria fungi protozoa virus 7 of 41 © Boardworks Ltd 2006
  8. 8. How do pathogens cause illness? Pathogens cause illness in three main ways: Toxins Toxins are harmful substances produced by the pathogen that poison the body’s tissue and enzymes. Reproduction A rise in the number of pathogens can damage a cell, even causing it to burst. Some pathogens hijack resources that the cell needs to survive. Immune response Sites of infection often become swollen, sore and hot as a result of increased blood flow. 8 of 41 © Boardworks Ltd 2006
  9. 9. How are pathogens spread? Different pathogens have different transmission routes: insect bites food and water direct contact 9 of 41 airborne droplets indirect contact © Boardworks Ltd 2006
  10. 10. Immunity to Infection
  11. 11. Fighting infection How does the body fight infection? 11 of 41 © Boardworks Ltd 2006
  12. 12. Immunity to Infection • Immunity is the acquired ability to defend against infection by disease-causing organisms.
  13. 13. Overview of your immune system • First line of defense: Physical barriers that viruses, bacteria must cross – skin covers ~2 m2 – Mucous membranes that line digestive, respiratory, reproductive tracts cover ~400 m2 • Second line of defense: Innate immune system (germline-encoded receptors -- no adaptation to specific pathogens) – Macrophages (Greek for “big eater”), neutrophils, natural killer (NK) cells • Third line of defense (vertebrates only): Adaptive immune system (adapts to defend against specific pathogens using variable receptors) – B cells make antibodies that vary -- can make an antibody specific for any new antigen – T cells mediate cellular responses using variable receptors (T cell receptors; TCRs)
  14. 14. First line of defence 14 of 41 © Boardworks Ltd 2006
  15. 15. Invasion! A pathogen enters your body through direct or indirect contact. What happens next? The pathogen begins to reproduce and make toxins that destroy the body’s cells and make you feel unwell. Painkillers can relieve the symptoms of an infection but do not kill the pathogen. Your immune system begins to mount an attack. 15 of 41 © Boardworks Ltd 2006
  16. 16. Second line of defence – Innate immunity Macrophages 16 of 41 © Boardworks Ltd 2006
  17. 17. Second line of defence - Natural killer (NK) cells • Can kill tumor cells, virus-infected cells, bacteria, parasites, fungi in tissues • Identify targets based on “missing self” – Two types of NK receptors: inhibitory and activating – If inhibitory receptor recognizes a self protein (a class I MHC molecule) on a target cell, the NK cell is turned OFF even if activating receptor binds a ligand on the same target cell – If activating receptor binds a ligand, but inhibitory receptor does not (target cell has down-regulated class I MHC proteins), NK cells kill – Many virally-infected cells and tumor cells down-regulate expression of class I MHC molecules (NK cells important for preventing cancers) 17 of 41 © Boardworks Ltd 2006
  18. 18. Resisting attack TB bacteria have a number of adaptations that enable them to resist phagocytosis:  They produce chemicals that prevent macrophages from being attracted to sites of infection.  They have a waxy cell wall that resists enzyme attack and secretes chemicals that block lysosomes from fusing with phagosomes. How does the immune system deal with these bacteria? 18 of 41 © Boardworks Ltd 2006
  19. 19. Third line of defence - Lymphocytes Lymphocytes are a type of white blood cell found in the blood or lymph nodes and made by bone marrow. There are several types of lymphocyte, including:  T-lymphocytes – recognise antigens on pathogens and either attack them directly or co-ordinate the activity of other cells of the immune system.  B-lymphocytes – recognise antigens and produce special chemicals called antibodies. 19 of 41 © Boardworks Ltd 2006
  20. 20. B lymphocytes Antibodies are special Y-shaped proteins produced by B-lymphocytes in response to antigens. Antibodies work by binding to antigens on pathogens, ‘labelling’ them and causing them to clump together. The pathogen can then be destroyed by:  phagocytosis by macrophages  T-lymphocytes  the antibodies themselves. 20 of 41 © Boardworks Ltd 2006
  21. 21. Antibodies Each different type of antigen causes a different type of antibody to be produced. An antibody can only bind to the antigen that caused it to be produced. 21 of 41 © Boardworks Ltd 2006
  22. 22. Delayed response The B-lymphocyte that produces the correct antibody for the antigen begins dividing to produce many more antibodyproducing cells. It takes a few days to produce enough antibodies to destroy the pathogen. This means there is delay between infection and the person beginning to feel better. Once a pathogen has been destroyed, a few memory cells remain. These recognize the pathogen if it re-infects, and make the immune response much quicker and more effective. This is called active immunity. 22 of 41 © Boardworks Ltd 2006
  23. 23. Antibody levels during infection 23 of 41 © Boardworks Ltd 2006
  24. 24. T cells How does the body deal with pathogens that are inside cells? Viruses and bacteria that infect cells leave antigens on the surface of the cell they infect. T-cell infected cell antigen receptor T-lymphocytes recognize these antigens by receptors on their surface and destroy the whole infected cell. 24 of 41 © Boardworks Ltd 2006
  25. 25. Types of Immunity Active immunity Passive immunity Naturally acquired Naturally acquired Artificially acquired Artificially acquired 25 of 41 © Boardworks Ltd 2006
  26. 26. Types of Immunity • Natural immunity is the result of a body’s previous encounter with an organism. • Artificial immunity results from the injection of a vaccine or an antibody. Vaccines stimulate active immunity whereas injection antibody or antiserum is an example of passive immunity. 26 of 41 © Boardworks Ltd 2006
  27. 27. Types of Immunity • Active immunity is when the immune system encounters and antigen and is primed to recognise it and destroy it quickly the next time it is encountered. This is active immunity because the body’s immune system prepares itself for future challenges. • Passive immunity is short-term and involves the transfer of immunity from one individual to another via antibody-rich serum. This may be artificial as is the case with anti-venom or natural, as in antibodies crossing the placenta to protect the developing foetus. 27 of 41 © Boardworks Ltd 2006
  28. 28. Immunity Active immunity Production of a person’s own antibodies. Long lasting Natural Active Artificial Active When pathogen Vaccination – usually enters body in the contains a safe antigen normal way, we from the pathogen. make antibodies Person makes antibodies without becoming ill Passive immunity An individual is given antibodies by another Short-term resistance (weeks- 6months) Natural Passive Baby in utero (placenta) Breast-fed babies Artificial Passive Gamma globulin injection Extremely fast, but short lived (e.g. snake venom) Edward Jenner 28 of 41 © Boardworks Ltd 2006
  29. 29. Passive immunity Many snakes produce a powerful nerve toxin that can be lethal to humans. People bitten by poisonous snakes can be treated with antivenin. Antivenin contains antibodies to give instant immunity. It is produced by injecting horses with small, non-lethal doses of venom. Over time, the horses produce antibodies, which are extracted and processed. Because the person didn’t make the antibodies themselves, this is called passive immunity. 29 of 41 © Boardworks Ltd 2006
  30. 30. What are vaccines? Vaccines contain a small amount of dead or weakened pathogen particles. A vaccine stimulates the production of antibodies and memory cells against the target pathogen, without making the person ill. If a vaccinated person is later infected by the same pathogen, their immune system can destroy it very quickly. Parents of two-year-old children are offered a combined measles, mumps and rubella (MMR) vaccine to protect their child. What has happened to MMR vaccination rates recently? 30 of 41 © Boardworks Ltd 2006
  31. 31. Measles, mumps and rubella MMR vaccination rates used to be high, but fell following a media scare story. The media reported on controversial research speculating that MMR could cause autism, a behavioural disorder causing learning and communication difficulties. 31 of 41 © Boardworks Ltd 2006
  32. 32. Glossary (1/2) antibiotic – A drug that can destroy or prevent the growth of bacteria. antibody – A Y-shaped protein produced by the body that binds to antigens. antigen – A substance on pathogens that stimulates the production of antibodies. B-lymphocyte – A white blood cell that produces antibodies. immunization – The process of protecting against infection by using a vaccine. immune response – The body’s defence against foreign material such as pathogens. 32 of 41 © Boardworks Ltd 2006
  33. 33. Glossary (2/2) immunity – The ability to fight infection by pathogens. It can be active or passive. pathogen – A disease-causing micro-organism. phagocytosis – The process where a type of white blood cell called a macrophage ingests and destroys a pathogen. T-lymphocyte – A type of white blood cell that recognises and destroys pathogens, and co-ordinates the immune response. tuberculosis – A serious bacterial disease that mainly affects the respiratory system. vaccine – A small amount of dead or weakened pathogen that stimulates antibody production. 33 of 41 © Boardworks Ltd 2006
  34. 34. Multiple-choice quiz 34 of 41 © Boardworks Ltd 2006
  35. 35. • Please read after going back • Things here are highly volatile 35 of 41 © Boardworks Ltd 2006