infection & response

• pathogens are microorganisms that cause infectious disease
• pathogens maybe viruses, protists, bacteria or fungi
• bacteria and viruses reproduce rapidly inside the body
• bacteria produces poisons (toxins) that damage tissues and make us feel ill
• viruses call illness by living and reproducing inside our cells, using our cell machinery to produce many copies of themselves. This causes our cells to burst, and causes cell damage
• protists cause illness by spreading through a vector. Protists are eukaryotes, and illness is spread when the infected vector bites or feeds on another organism
• fungi cause illness through their body which is made up of hyphae. The hyphae can penetrate human skin causing disease, and produces spores which allows the fungi to spread
• spread through water -> drinking or bathing in dirty water
• spread through direct contact -> touching a contaminated surface or someone with the disease
• spread through air -> breathing in infected cough and sneeze droplets
• virus : measles --

  • affects children
  • symptoms are a fever and red skin rash, and can be fatal if complications arise
  • spread through the inhalation of infected droplets from coughs and sneezes
  • children are vaccinated against measles through the MMR vaccine
• virus : HIV --

  • spread through sexual contact or sharing of bodily fluids such as blood which occurs when drug users share needles
  • HIV initially causes flu-like symptoms. Unless successfully controlled with antiretroviral drugs, the virus attacks the body's immune system.
  • Late stage HIV infection (or AIDS) occurs when the body's immune system becomes so badly damaged it can no longer deal with other infections or cancers
• virus : tobacco mosaic virus --

  • widespread plant pathogen that affects many species of plants including tomatoes
  • gives a distinct 'mosaic' pattern of discolouration on the leaves
  • discolouration affects the growth of the plant because it means less absorption of sunlight, less photosynthesis and less glucose and respiration to grow
• bacterial diseases : salmonella:

  • salmonella food poising is spread by bacteria ingested in food, or food prepared in unhygienic conditions
  • symptoms include fever, abdominal cramps, vomiting and diarrhoea caused by the bacteria and the toxins they secrete
  • in the UK, all poultry are vaccinated against Salmonella to control the spread
• bacterial diseases : gonorrhoea --

  • sexually transmitted disease, spread by sexual contact
  • symptoms include thick green or yellow discharge from the penis or vagina, and pain urinating.
  • it is caused by a bacterium, and was easily treated with the antibiotic penicillin until many resistant strain appeared
  • the spread can be controlled by treatment with antibiotics or the use of barrier methods of contraception such as condoms
• fungal disease : rose black spot --

  • spread in the environment by wind or water
  • symptoms are purple and black spots that develop on leaves, which often turn yellow and drop early. It affects the growth of the plant as photosynthesis is reduced
  • can be treated using fungicides and removing and destroying the affected leaves
• protist disease : malaria --

  • the pathogens that cause malaria are protists
  • the malarial protist has a life cycle that includes the mosquito
  • malaria causes recurrent episodes of fever and can be fatal
  • the spread of malaria is controlled by preventing the vector (mosquitoes) from breeding so not leaving areas of open still water and using mosquito nets to avoid being bitten
• non-specific defences : skin --

  • acts as a physical barrier
  • gland secrete antiseptic oils to kill pathogens
  • good microorganisms (skin flora) compete with bad microorganism for space and nutrients
• non-specific defences : the nose --

  • nose hairs that act as a physical barrier by trapping pathogens
  • mucus traps pathogens and dust from entering the lungs
• non-specific defences : trachea and bronchi --

  • secrete mucus to trap pathogens
  • ciliated cells waft mucus upwards to be swallowed, and enter the stomach
• non-specific defences : stomach --

  • produces hydrochloric acid that kills any pathogens swallowed in food or in the mucus
• the immune system protects the body against pathogens
• if a pathogen enters the body, the immune system tries to destroy the pathogen
• white blood cells : phagocytosis --

  • phagocytes engulf pathogens
  • this destroys them, preventing them from making you feel ill or reproducing
• white blood cells : producing antibodies --

  • white blood cells produce antibodies that are complimentary to the antigen on pathogens
  • antibodies bind on to pathogens, meaning the pathogen can no longer reproduce or cause harm
  • the pathogens start to clump together, and then a phagocyte engulfs them
  • the antigen and specific antibody is stored in memory cells so if the same pathogen reenters, more antibodies are produced faster leading to an improved immune response
• white blood cells : producing antitoxins --

  • white blood cells produce antitoxins which neutralise the toxins produced by bacteria
• vaccination:

  • small quantities of a dead or inactive form of the pathogen is injected
  • the body recognises this pathogen as foreign and stimulates the white blood cell to produce antibodies complimentary to the antigen
  • memory cells store the antigen and the specific complimentary antibody
  • if the same pathogen reenters the body, the memory cells will recognise it, and the white blood cells will produce more of the correct antibodies quicker.
  • this leads to an improved immune response, preventing infection
• by immunising a large proportion of the population, the spread of pathogens is reduced as there are less people to catch the disease from (herd immunity)
• antibiotics are drugs that kill bacterial pathogens inside the body, without damaging body cells
• antibiotics cannot kill viral pathogens because viruses use our body cells to replicate meaning the drug would have to damage our cells to work
• antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. It is important that specific bacteria should be treated by specific antibiotics.
• the use of antibiotics has greatly reduced the deaths from infections bacterial diseases, however the emergence of resistant strains is of great concern
• painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens.
• it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.
• antibiotic resistance:

  1. a bacterial population, one bacterium will have a DNA mutation, meaning it is resistant to an antibiotic
  2. when the antibiotic is taken, the susceptible bacteria are killed
  3. this reduces competition for nutrients and space meaning the antibiotic resistant bacteria reproduce rapidly, forming a large population that is difficult to control
• developing drugs:

  1. drug is tested on tissues in a lab to make sure it isn't toxic, and efficacy
  2. the drug is tested on animals such as mice to check its toxicity and for side effects
  3. drug is given to healthy volunteers at a low dose to check the toxicity, and if there are any unwanted side effects
  4. drug is given to volunteer patients, starting at a low dose to find the optimum dose
  5. a double-blind trial is conducted with patients to eliminate bias and check efficacy
  6. apply to government and international health organisations for permission to market, prescribe and sell the drug
• traditionally drugs were extracted from plants and microorganisms:

  • the heart drug digitalis originates from foxglove
  • painkiller aspirin originates from willow bark
  • penicillin was discovered by Alexander Fleming from the penicillium mould which developed a clear ring when growing on a bacterial petri-dish
• most new drugs are synthesised by chemists in the pharmaceutical industry, however the starting point may still be a chemical extracted from a plant
• new medical drugs have to be tested and trialled before being used to check that they are safe and effective.
• new drugs are extensively tested for toxicity, efficacy and dose.