Infection and response

• Pathogens


  Microorganisms that cause infectious diseases
• Bacteria


  • Reproduce very rapidly inside human body
  • Release harmful chemicals called toxins which damage tissues and make us feel ill
• Viruses


  • Can't reproduce by themselves, only reproduce inside a host cell
  • First the virus invades the host cell, then reproduces inside host cell
  • Damages the cell, when the virus leaves it can cause the cell to burst open and die
• How pathogens are spread

  • By air (water droplets)
  • Directly in water
  • Direct contact between individuals (HIV)
• Viruses can't be killed by antibiotics
• Measles


  • First symptom is a fever
  • After 3 days, develop a red skin rash
  • Sometimes causes damage to breathing system and the brain
• HIV


  • First symptom is a flu like illness where the virus is attacking the cells of patient's immune system
  • Sometimes immune system is so damaged that it can't fight off any other infections
  • Antiretroviral drugs stop the virus from multiplying inside the patient. Not a cure, must take them for the rest of their lives
• Bacteria can be killed by antibiotics
• Salmonella


  • The bacteria secrete harmful chemicals called toxins which cause the symptoms
  • Symptoms: fever, abdominal cramps, vomiting and diarrhoea
• Gonorrhoea


  • It's a sexually transmitted disease (STD)
  • Symptoms: thick yellow/green discharge from vagina or penis and pain when urinating
• Malaria


  • Communicable, spread by the pathogen protist's (mosquito)
  • Experience repeated bouts of fever
  • Mosquito is a vector
  • To prevent the spread of malaria we must: stop the vector from breeding, prevent mosquito's from biting humans (use mosquito net)
• Non-specific defence systems


  • The job is to prevent pathogens from entering the human body
• Skin


  • Forms protective layer covering body
  • Outer layer of skin consists of dead cells and is difficult for pathogens to penetrate
  • Also produces an oily substance called sebum which kills bacteria
  • Damaged skin, skin scabs over to prevent pathogens from entering
• Nose


  • Contains hair and mucus to trap pathogens before entering breathing system
• Lungs


  • Trachea and bronchi are covered with tiny hairs called cilia
  • Cilia are covered in mucus which traps pathogens
• Stomach


  • Contains hydrochloric acid which kills pathogens before they can go to the digestive system
• The immune system


  • Destroys pathogens and any toxins they produce
  • Protects us in case the same pathogen invades again
• White blood cells


  • Ingest and destroy pathogens using enzymes. Called phagocytosis
  • Making antibodies which destroys pathogens. The antibodies are extremely specific and can remain in the blood for a long time
  • Produce antitoxins
• Tobacco mosaic virus (TMV)


  • Causes leaves to discolour in a mosaic pattern
  • Causes rate of photosynthesis is reduced, this means growth of the plant is also reduced
• Rose black spot


  • Caused by fungus can spread by water or wind
  • Causes leaves to develop purple or black spots. Turn yellow and fall of after
  • Also causes rate of photosynthesis to fall
  • Treatment: Spray plants with chemicals that kill fungi (fungicides) and remove infected leaves and destroy them
• Vaccination


  • Involves introducing small quantities of dead and inactive forms of a pathogen into the body
  • Because the pathogen is dead or inactive, it can't lead to the disease in the patient
  • White blood cells stimulated to produce antibodies and white blood cells divide by mitosis
• Herd immunity


  A large group is vaccinated so it protects those who aren't vaccinated as well because no one can spread it
• Antibiotics


  • Kill bacteria
  • Antibiotic resistance: some bacteria don't die with antibiotics because they had evolved because of overuse of antibiotics
  • Doctors always use specific antibiotics, they don't kill viruses, painkillers treat symptoms
• Testing medicines


  • Plant foxglove was used to extract the heart drug digitalis
  • Painkiller aspirin was extracted from willow trees
  • Drugs were also extracted from microorganisms. Alexander Fleming discovered penicillin in the mould penicillium
  • Check if the drugs safe for humans, effective, work out best dose of drug
  • Preclinical testing is not carried out on humans it's tested on cells, tissues and live animals
  • Clinical testing is carried out on humans. First stage, very low doses given to healthy volunteers, if safe carry on to find best dose with fewest side effects
  • Placebo is a tablet or injection with no active drug in it, think they get better
• Double-blind trial


  • Test group receive active drug
  • Placebo group receive dummy drug
  • Doctors and patients don't know who received the real or placebo drug to stop bias