Diseases and immunity

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• CIE IGCSE Biology
• Contents


  • 10.1.1 Pathogens & Barriers
  • 10.1.2 Controlling the Spread of Disease
  • 10.1.3 Active Immunity: Extended
  • 10.1.4 Antigens & Antibodies: Extended
  • 10.1.5 Vaccination: Extended
  • 10.1.6 Preventing the Spread of Disease: Extended
  • 10.1.7 Passive Immunity & Breastfeeding
  • 10.1.8 Cholera
• Pathogen


  A disease-causing organism
• Transmissible diseases


  Diseases that can be passed on from one host to another
• Ways pathogens can be passed on from host to host

  • Direct contact
  • Indirect contact
• There are 3 main ways in which the body defends itself against disease
• Body defences


  • Mechanical barriers
  • Chemical barriers
  • Cells
• Mechanical barriers


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


  • Covers almost all parts of your body to prevent infection from pathogens
  • If it is cut or grazed, it immediately begins to heal itself, often by forming a scab
• Hairs in the nose

  • Make it difficult for pathogens to get past them further up the nose so they are not inhaled into the lungs
• Chemical barriers


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


  • Made in various places in the body, pathogens get trapped in the mucus and can then be removed from the body (by coughing, blowing the nose, swallowing etc)
• Stomach acid


  • Contains hydrochloric acid which is strong enough to kill any pathogens that have been caught in mucus in the airways and then swallowed or have been consumed in food or water
• 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

  • By phagocytosis - engulfing and digesting pathogenic cells
  • By 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

  • Good hygiene
  • Effective sanitation and waste disposal
• Active immunity is slow acting and provides long-lasting immunity
• 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
• Antigen


  Molecules, such as proteins, projecting from cell membranes
• Lymphocytes can recognise foreign antigens, e.g. the antigens of a pathogen inside the body
• In response to foreign antigens lymphocytes make antibodies which are complementary in shape to the antigens on the surface of the pathogenic cell
• Antibodies


  Proteins made by lymphocytes that are complementary to an antigen and, when attached, clump them together and signal the cells they are on for destruction
• Antibodies can attach to antigens and cause agglutination of pathogens
• Agglutinated pathogens cannot move around so phagocytosis can occur more easily
• Immunity


  The initial response of a lymphocyte encountering a pathogen for the first time can take a few days, during which time an individual may get sick
• After an initial encounter with a pathogen lymphocytes can give rise to memory cells that retain the instructions for making specific antibodies
• This means that, in the case of reinfection by the same type of pathogen, antibodies can be made very quickly and in greater quantities; the pathogens are destroyed before they multiply and cause illness
• This is how people become immune to certain diseases after only having them once
• Vaccines allow a dead or altered form of the disease-causing pathogen, which contains specific antigens, to be introduced into the body
• In this weakened state, the pathogen cannot cause illness but can provoke an immune response
• Lymphocytes produce complementary antibodies for the antigens
• The antibodies target the antigen and attach themselves to it in order to create memory cells
• The memory cells remain in the blood and will quickly respond to the antigen if it is encountered again in an infection by a 'live' pathogen
• If a large enough percentage of the population is vaccinated, it provides protection for the entire population because there are very few places for the pathogen to breed - it can only do so if it enters the body of an unvaccinated person
• Herd immunity


  Protection for the entire population because there are very few places for the pathogen to breed