Disease, Defence & Treatment

• Main types of pathogen

  • Virus
  • Bacterium
  • Fungus
  • Protoctista
• Pathogen life cycle


  1. Infect a host

  2. Reproduce themselves

  3. Spread from their host

  4. Infect other organisms
• Pathogens have structural adaptations that make them successful at completing their life cycle, which enables them to cause further disease
• Communicable diseases


  Diseases caused by pathogens that can be transferred from one person to another (contagious)
• Viruses are not considered alive because they do not complete all of the seven life processes: movement, respiration, sensitivity, nutrition, excretion, reproduction and growth
• Virus particle parts


  • DNA or RNA
  • Envelope
  • Protein coat
• Ways diseases can spread

  • Direct contact - Sexual contact or non-sexual contact
  • Water - Dirty water can transmit many diseases
  • Sneezing - When a person who is infected by the common cold sneezes, they can spray thousands of tiny droplets containing virus particles to infect others
  • Unhygienic food preparation - Undercooked or reheated food can cause bacterial diseases like E.coli, which is a cause of food poisoning
  • Vector - Any organism that can spread a disease is called a vector.
• HIV/AIDS


  HIV is often transmitted by body fluids, often during unprotected sex, but also through cuts and injecting drugs using shared needles. Months or years after infection by the HIV virus, it becomes active and starts to attack the patient's immune system, at which point HIV has become AIDS.
• Skin


  • If it's cut or grazed, it immediately begins to heal itself, often by forming a scab. This prevents infection as the skin acts as a physical barrier.
• Nose


  • Has internal hairs, which act as a physical barrier to infection.
  • Cells in the nose produce mucus which traps pathogens before they can enter the lungs.
  • When the nose is blown, the mucus, and any pathogens that are trapped with it, is removed.
• Trachea and Bronchi


  • The trachea runs from the larynx to the bronchi.
  • The cells that line the trachea have hairs called cilia, which waft mucus and pathogens upwards towards the throat where they are swallowed into the stomach.
  • Goblet cells create the mucus in order to trap pathogens.
  • The production of mucus in the airways is a physical barrier.
• Stomach


  • Stomach acid is part of the body's non-specific first line of defence. It contains hydrochloric acid, which is strong enough to kill any pathogens that have been caught in mucus.
• Types of white blood cells


  • Phagocytes
  • Lymphocytes
• How phagocytes work


  1. Microbes have proteins on their surface called antigens.

  2. The phagocytes recognise the antigens on the microbe as an invader.

  3. The phagocyte engulfs the microorganism.

  4. Enzymes inside the phagocyte digest the microorganism.
• How lymphocytes work


  1. Lymphocytes detect that antigens are foreign and produce antibodies as a result.

  2. The antibodies cause pathogens to stick together and make it easier for phagocytes to engulf them.

  3. Some pathogens produce toxins. Lymphocytes can produce antitoxins to neutralise these toxins.

  4. If the body encounters a particular antigen, lymphocytes might recognise it and clone themselves to make enough antibodies to destroy the pathogen. Memory cells are also created which remain in the bloodstream and produce a quick response if the antigen is encountered again.
• Vaccines


  Allow dead or altered forms of a disease-causing pathogen, which contain a specific antigen, to be introduced into the body. This causes the immune system, specifically the white blood cells, to produce complementary antibodies which target the antigen and attach to it in order to create memory cells.
• How vaccines work


  1. Syringe injects an altered form of the pathogen.

  2. White blood cells release complementary antibodies to the specific antigen.

  3. They attach and clump pathogens together.

  4. White blood cells engulf the pathogens through phagocytosis.
• Antibiotics


  Substances that slow down or stop the growth of bacteria. Examples include penicillin and amoxicillin. They can be taken to cure bacterial diseases by killing the pathogen, but are not effective against viral diseases.
• Some antibiotic resistant bacteria such as MRSA results from an overuse of antibiotics. Good hygiene like hand washing, alcohol gels, effective cleaning of hospital wards etc. need to be used to prevent the spread.
• Stages of drug development

  1. Preclinical - testing on human cells grown in the laboratory, testing on animals, testing on healthy human volunteers.

  2. Clinical - testing on small groups of patients. Placebo used instead of a drug in a drug trial.

  3. Blind trial - patients do not know if they have been given the drug or placebo but doctors know.

  4. Double blind trial - neither patients or doctors know, only researchers know.
• Monoclonal antibodies


  Identical copies of one type of antibody. Formed by injecting an antigen into a mouse, removing the spleen cells that produce the antibodies, and fusing them with cancerous white blood cells to create hybridoma cells that divide indefinitely and produce millions of monoclonal antibodies specific to the original antigen.
• Uses of monoclonal antibodies

  • Immunoassays - MAb are made for specific antigens on disease causing agents and labelled to detect the extent of infection.
  • Tissue Typing - MAb can be used against helper T-cells so B-lymphocytes, normally causing rejection, are prevented from functioning.
  • Treating cancers - MAb can be made for cancer cell antigens (tumour markers) and carry anti-cancer drugs directly to the cells.
  • Monitoring the spread of malaria - MAb specific to Plasmodium antigens are used to test blood samples to detect the spread of malaria and the effectiveness of anti-malaria drugs.