Immune System

• the body has three lines of defence:

  • external non-specific
  • internal non-specific
  • internal specific
• communicable diseases


  contagious diseases that can be spread from one person to another by direct, indirect or droplet contact
• pathogen


  a disease causing organism

  • bacteria
  • virus
  • fungi
  • animal parasites
• bacteria

  • very small single cell structured organism that has a cell wall
  • prokaryotic cell that contains DNA
  • can multiply rapidly
  • can have different shapes - rod, spherical, etc.
  • treated with antibiotics which kills bacterial infections
  • wash hands with antibacterial soap before and after preparing food and after using the toilet
  • e.g. chlamydia, gonorrhoea, tetanus, whooping cough, food poisoning
• viruses

  • smaller than bacteria
  • not a living cell
  • contains genetic material (DNA or RNA)
  • contains a protective protein coat internally that is hard to penetrate and kill
  • some have external lipid membrane
  • the virus can't kill and multiply on its own so must invade a host cell - bacteriophage
  • cannot be killed so symptoms are treated - antiviral treatment
  • e.g. HIV/AIDS, chickenpox, cold sores, colds, genital herpes, measles, rabies, warts
• Transmission of pathogens


  • Contact
  • Ingestion
  • Transfer of Body fluids
  • Droplets
  • Airborne
  • Vectors
• Transmission by contact


  • Skin infections
  • STIs
• Direct physical contact


  Physically touching an infected person
• Indirect physical contact


  Touching an object that has been touched by an infected person
• Ingestion


  Consuming food and drink contaminated with pathogens
• Pathogens ingested


  • Salmonella
  • Typhoid
• Transfer of body fluids


  1. From an infected person to an uninfected person

  2. Through mucous membranes, nose, mouth, throat, genitals
• Infection by droplets


  1. Tiny droplets of moisture containing a pathogen

  2. Emitted through coughing, sneezing, talking, breathing

  3. Land on food or utensils which could be ingested
• Airborne transmission


  1. Droplets exhaled through talking or coughing

  2. Pathogens can cause infection when inhaled
• Transmission by vectors


  1. Transfer of pathogens using animals such as fleas or ticks

  2. Transfer by a fly onto food which is ingested
• external non-specific first line of defence

  • skin
  • mucus
  • hairs/cilia
  • acid
  • lysozyme
  • cerumen
  • movement of fluid
• skin

  • impervious structure (physical barrier) that is hard to penetrate and prevents entry of pathogens
  • contains oily sectretion called sebum which kills some bacteria and prevents cracking of skin
  • sweat contains salts and fatty acid which prevent growth of pathogens
• mucus

  • mucous membranes are located in the digestive, urinary and reproductive systems
  • traps pathogens, inhibits entry of micro-organism and pushes pathogens out of the body
• hairs/cilia

  • found in nasal cavity, ear and throat
  • trap particles when inhaled
  • cilia in nasal cavity and trachea have mucus which traps the micro-organism, the cilia beat and move the mucus upwards and out of the body
• acid

  • found in the stomach
  • destroys pathogens in food or inhibits/prevents growth of pathogen
  • found in vagina which secretes acid to reduce growth of micro-organisms
  • urine/sweat on the skin are also slightly acidic
• lysozyme

  • an enzyme found inside tears, saliva, nose fluid and sweat
  • destroys bacteria/pathogens
• cerumen

  • ear wax which protects outer ear from bacteria
  • slightly acidic and contains lysozyme
• movement of fluid

  • flushing fluids to keep areas free of pathogens - urine through urethra prevents bacterial growth
  • tears, sweat, and saliva are involved in flushing and cleansing
• protective reflexes

  • sneezing
  • coughing
  • vomiting
  • diarrhoea
• sneezing

  • stimulus -irritation of the walls of the nasal cavity e.g. noxious fumes or dust particles
  • forceful expulsion of air from the lungs carrying mucus, foreign particles and irritating gases out through the nose and mouth
• coughing

  • stimulus - irritation of the lower respiratory tract - bronchi and bronchioles
  • air is forced from the lungs to try to remove the irritant
  • air drives mucus and foreign matter up the trachea towards the throat and mouth
• vomiting

  • psychological stimuli, excessive stretching of the stomach or bacterial toxins can induce vomiting
  • contraction of the muscles of the abdomen and the diaphragm, not the contraction of the stomach, expels the stomach contents
• diarrhoea

  • irritation of the small or large intestines by bacteria, viruses or protozoans
  • increased contractions of the walls of intestines so irritant is removed so fast that water cannot be absorbed causing watery faeces
• phagocytes act if pathogen passes external defences
• monocytes and macrophages are part of the internal non-specific defence

  • monocytes leave the bloodstream and enter infected or inflamed tissue then differentiate into macrophages - large phagocytic cells
  • some move through tissues looking for and removing pathogens whilst others stay still and attack pathogens that come to them
  • important for removing microbes and dying cells through phagocytosis
• neutrophils are part of the internal non-specific defence

  • granulated leucocyte - granules visible in cytoplasm
  • lobulated nucleus
  • 55-70% of leucocytes
  • first cells to move into tissue and destroy pathogen by phagocytosis - kill pathogen inside cells
  • die quickly - after a few days they form pus
• dendritic cells are part of the internal non-specific defence

  • have projections from the cytoplasm
  • detect, engulf and process foreign particles then use this information for specific immunity
• The internal non-specific defence of the inflammatory response has the purpose of reducing the spread of any pathogens, to destroy and prevent entry of additional pathogens, to remove damaged tissue debris, and to begin the repair of damaged tissue and increase the speed of healing
• the four signs of inflammation are :

  • redness
  • swelling
  • pain
  • heat
• Inflammatory response


  Occurs when tissue is ruptured or damaged
• Inflammatory response


  1. Damaged cells stimulate release of mast cells

  2. Mast cells release histamine and heparin

  3. Histamine increases blood flow and causes vasodilation

  4. Increased blood flow causes swelling

  5. Histamine tells body's defence mechanisms to react

  6. Heparin prevents blood clotting

  7. Chemicals released by mast cells attract phagocytes

  8. Macrophages and leucocytes consume micro-organisms and debris via phagocytosis

  9. Pain receptors stimulated by abnormal conditions

  10. Phagocytes filled with bacteria, debris and dead cells begin to die

  11. Dead phagocytes and tissue fluid form pus

  12. New cells produced by mitosis to repair damaged cells
• Histamine


  Chemical released by mast cells that increases blood flow and causes vasodilation as well as telling natural defence mechanisms how to react to something it perceives as foreign
• Heparin


  Chemical released by mast cells that prevents blood clotting
• Phagocytes


  Macrophages and leucocytes that actively consume micro-organisms and debris
• Pus


  Mixture of dead phagocytes and tissue fluid