Disease and immunity

• Pathogen


  A disease-causing organism
• Pathogens are passed on from one host to another and therefore the diseases they cause are known as transmissible diseases
• Ways pathogens can be passed on from host to host

  • Direct contact
  • Indirect contact
• Direct contact


  The pathogen is passed directly from one host to another by transfer of body fluids such as blood or semen
• Diseases spread by direct contact

  • HIV
  • Gonorrhoea
  • Hepatitis B & C
• Indirect contact


  The pathogen leaves the host and is carried in some way to another, uninfected individual
• Methods of transmission


  • Droplets in air
  • Food or water
  • Touching contaminated surfaces
  • Insect bites
• Diseases spread by droplets in air

  • Common cold
  • Influenza
• Diseases spread by food or water

  • Cholera
  • Typhoid
  • Dysentery
• Diseases spread by touching contaminated surfaces

  • Athletes foot
  • Salmonella
• Diseases spread by insect bites

  • Malaria
  • Dengue fever
• Body defences against disease

  • Mechanical barriers
  • Chemical barriers
  • Cells
• Mechanical barriers


  Structures that make it difficult for pathogens to get past them and into the body
• Mechanical barriers


  • Skin
  • Hairs in the nose
• Chemical barriers


  Substances produced by the body cells that trap / kill pathogens before they can get further into the body and cause disease
• Chemical barriers


  • Mucus
  • Stomach acid
• Cells


  Different types of white blood cell work to prevent pathogens reaching areas of the body they can replicate in
• How cells defend against pathogens

  1. Phagocytosis - engulfing and digesting pathogenic cells

  2. Producing antibodies - which clump pathogenic cells together so they can't move as easily (known as agglutination) and releasing chemicals that signal to other cells that they must be destroyed
• The simplest way to prevent disease is to stop pathogens from spreading
• Measures to prevent the spread of disease

  • Hygienic food preparation
  • Personal hygiene
  • Waste disposal
  • Sanitation
• Hygienic food preparation


  • Keep food cold so bacteria and fungi reproduce more slowly
  • Prepare food hygienically to avoid contamination from pathogens by washing hands well with soap and cleaning work surfaces with products such as bleach to kill pathogens
  • Cook food well (long enough at high temperature) to kill bacteria and fungi
  • Cover food to prevent flies landing on it before eating
  • Use separate chopping boards/utensils for cutting uncooked meat
  • Wash hands after using the bathroom before handling food
• Personal hygiene


  • Washing with soap removes substances which trap pathogens as well as pathogens themselves from the skin
  • Use tissues to catch sneezes and coughs
  • Dispose of used tissues as soon as possible as pathogens can still be alive
  • Wash hands after using the bathroom
• Waste disposal


  • Waste food is a food source for flies that can act as vectors for transmissible diseases and so should be disposed of in a sealed container
  • Rubbish bins should be covered and removed to the landfill for disposal or burning regularly
  • All rubbish should be stored before collection away from human habitation
• Sanitation


  • Homes and public places should have plumbing and drains to safely remove faeces and waste which can carry pathogens
  • Raw sewage should be treated to remove solid waste and kill pathogens before being released into the environment
• Active immunity


  Making antibodies and developing memory cells for future response to infection
• Ways active immunity develops

  • The body has become infected with a pathogen and so the lymphocytes go through the process of making antibodies specific to that pathogen
  • Vaccination
• Active immunity is slow acting and provides long-lasting immunity
• Antigen


  Molecules projecting from cell membranes
• Different individuals have different antigens on their cell surface membranes
• Lymphocytes


  Can recognise foreign antigens, e.g. the antigens of a pathogen inside the body
• Lymphocyte response to foreign antigens

  Lymphocytes make antibodies which are complementary in shape to the antigens on the surface of the pathogenic cell
• Each lymphocyte only produces one type of antibody
• Antibodies


  Can attach to antigens and cause agglutination of pathogens, signalling the presence of pathogens to phagocytes
• Antibodies cause agglutination


  Pathogens cannot move around very easily
• Agglutination of pathogens


  Phagocytosis can occur more easily
• Immunity

  Initial response to a pathogen can take a few days, but lymphocytes can give rise to memory cells that retain instructions for making specific antibodies, allowing for a quicker and greater antibody response upon reinfection
• Immunity does not work with all disease-causing microorganisms as some mutate quickly and change their antigens
• Antigen


  A molecule found on the surface of a cell
• Antibody


  A protein made by lymphocytes that is complementary to an antigen and, when attached, clumps them together and signals the cells they are on for destruction
• Antitoxin


  A protein that neutralises the toxins produced by bacteria