Infection and Response

• Pathogens

  Microorganisms that cause infectious disease
• Types of pathogens

  • Viruses
  • Bacteria
  • Protists
  • Fungi
• Pathogens

  • May infect plants or animals
  • Can be spread by direct contact, by water or by air
• Bacteria

  • May reproduce rapidly inside the body
  • May produce poisons (toxins) that damage tissues and make us feel ill
• Viruses

  Live and reproduce inside cells, causing cell damage
• Reducing the spread of pathogens

  1. Sterilising water

  2. Preparing food hygienically

  3. Washing

  4. Vaccination

  5. Barrier contraception
• Measles

  • Viral disease showing symptoms of fever and a red skin rash
  • Serious illness that can be fatal if complications arise
• Measles virus


  Spread by inhalation of droplets from sneezes and coughs
• HIV

  • Initially causes a flu-like illness
  • Unless successfully controlled with antiretroviral drugs the virus attacks the body's immune cells
  • 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
• HIV

  Spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles
• Tobacco mosaic virus (TMV)


  • Widespread plant pathogen affecting many species of plants including tomatoes
  • Gives a distinctive 'mosaic' pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis
• Salmonella food poisoning

  • Spread by bacteria ingested in food, or on food prepared in unhygienic conditions
  • Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete
• Salmonella

  In the UK, poultry are vaccinated against to control the spread
• Gonorrhoea


  • Sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the vagina or penis and pain on urinating
  • Caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared
• Gonorrhoea


  • Spread by sexual contact
  • Spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as a condom
• Rose black spot


  • Fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early
  • Affects the growth of the plant as photosynthesis is reduced
• Rose black spot


  • Spread in the environment by water or wind
  • Can be treated by using fungicides and/or removing and destroying the affected leaves
• Malaria


  • Caused by protist pathogens
  • Has a life cycle that includes the mosquito
  • Causes recurrent episodes of fever and can be fatal
• Malaria

  Spread is controlled by preventing the vectors, mosquitos, from breeding and by using mosquito nets to avoid being bitten
• White blood cells

  • Help to defend against pathogens by: phagocytosis, antibody production, antitoxin production
• Types of white blood cells

  • Phagocytes
  • Lymphocytes
• Phagocytes

  Engulf and digest pathogens, this can be non-specific or helped by antibodies which cause agglutination (clumping) of pathogens
• Lymphocytes


  • Produce antibodies
  • Produce antitoxins

  
• Antibodies


  • Y-shaped proteins
  • Each individual has the potential to make millions of different types, each with a slightly different shape
  • Aim is to produce the antibody that is specific (complementary) to the antigens on the surface of the pathogen

  
• Memory cells


  • Lymphocytes that remain in the body after an initial infection with a particular pathogen
  • Produce the specific antibodies against its antigens so that if you get infected by the same pathogen again in the future you can produce antibodies much quicker

  
• Antitoxins


  Proteins that neutralise the toxins produced by bacteria
• Vaccination


  Involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies

  
• Immune response to vaccination

  1. White blood cells multiply in number and produce lots of antibodies which are specific and complementary to the pathogen

  2. Creates memory cells

  3. If the pathogen enters the individual's body again the immune system recognises it and quickly produces the correct antibodies so that the pathogen is removed before the individual shows any symptoms, avoiding illness

  
• Herd immunity


  Achieved when enough of the population is vaccinated against a certain pathogen, making it unlikely the pathogen will enter a person who has not been immunised
• Antibiotics


  • Medicines that help to cure bacterial disease by killing infective bacteria inside the body
  • Important that specific bacteria should be treated by specific antibiotics
  • Use of antibiotics has greatly reduced deaths from infectious bacterial diseases
• Antibiotics cannot kill viral pathogens
• 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
• Clinical trials for new drugs

  1. Preclinical Testing - Lab Testing

  2. Preclinical Testing - Animal Testing

  3. Clinical Testing - Phase 1

  4. Clinical Testing - Phase 2

  5. Clinical Testing - Phase 3
• Preclinical Testing - Lab Testing


  • Involves computer modelling and testing outside of the body in vitro
  • Aims to make sure the drug is safe and effective before introducing it into animals or humans
• Preclinical Testing - Animal Testing


  Involves testing the drug on animals normally small mammals such as mice, to ensure the drug is safe for consumption
• Clinical Testing - Phase 1

  • Uses a very low dose
  • Number of patients is quite small (15-20 participants)
  • Aim is to check if the treatment is safe and to find the right dose
• Clinical Testing - Phase 2


  • Tests how well the drug works in actually treating patients - the efficacy
  • Larger test involving more participants (20-150 participants)
• Clinical Testing - Phase 3


  • Comparison of the drug to the current treatment in a large trial
  • Can involve 100s or 1000s of participants
  • Some trials are double blind where some patients are given a placebo
• Monoclonal antibodies


  • Produced from a single clone of cells
  • Specific to one binding site on one protein antigen and so able to target a specific chemical or specific cells in the body
  • Produced by stimulating mouse lymphocytes to make a particular antibody