HB - Chapter 7

• Infectious diseases caused by invasion of pathogens in the form of viruses and bacteria can be transmitted from one host to another
• Transmission of pathogens occurs by various mechanisms

  • Direct and indirect contact
  • Transfer of body fluids
  • Disease-specific vectors
  • Contaminated food and water
• The body's external defence mechanisms against pathogens include features of


  • Skin
  • Digestive tract
  • Urogenital tract
  • Respiratory system
  • The ear
  • The eye
• Pathogens that enter the body are targeted by non-specific immune responses of inflammation and fever
• Immunity


  Gained through the exposure to specific antigens by the production of antibodies by B lymphocytes and the provision of cell-mediated immunity by T lymphocytes; in both cases memory cells are produced
• Passive immunity

  Acquired as antibodies gained through the placenta, or antibody serum injections
• Active immunity

  Acquired through natural exposure to the pathogen, or the use of vaccines
• Antiviral and antibiotic drugs

  Used for treating infections and differ in their specificity to pathogens
• Pathogen


  Micro-organism such as bacteria and viruses that can cause disease
• Transmission of pathogens


  From infected person to someone else, either directly or indirectly
• During the COVID-19 pandemic, people were advised to wash their hands with soap


  This would be effective in preventing the transmission of the coronavirus by breaking down its protein coat
• There is some debate as to whether viruses are living things
• It is easier to stop the transmission of pathogens that are transferred by body fluids


  Than those that are transferred by moisture
• Non-specific defences


  Work against all pathogens, the body's first line of defence
• Specific defences


  Directed at a particular pathogen
• External defences


  • Skin
  • Mucus
  • Hairs
  • Cilia
  • Acids
  • Lysozyme
  • Cerumen
  • Movement of fluid
• The skin is an effective barrier covering the outside of the body, stopping the entry of micro-organisms provided it is not broken
• Huge numbers of bacteria live on the skin all the time, occupying the area so potential pathogens find it difficult to become established
• Sebum produced by oil glands in the skin contains substances that kill some pathogenic bacteria
• Sweat secreted on to the skin contains salts and fatty acids that prevent the growth of many micro-organisms
• Mucous membranes secrete mucus, which traps particles and inhibits the entry of micro-organisms to the organs of the body
• Hairs in the nasal cavity trap up to 90% of particles inhaled when breathing
• The beating of cilia moves mucus, containing trapped particles and micro-organisms, towards the throat, where it may be coughed up or swallowed
• Stomach juices are strongly acidic, killing many of the bacteria taken in with food or those contained in mucus swallowed from the nose and windpipe
• The vagina also has acid secretions that reduce the growth of micro-organisms
• Urine and the sweat on the skin are also slightly acidic
• Lysozyme is an enzyme that kills bacteria, found in tears, saliva, sweat, secretions of the nose and tissue fluid
• Cerumen, or ear wax, protects the outer ear against infection by some bacteria, being slightly acidic and containing lysozyme
• The flushing action of body fluids helps to keep some areas relatively free of pathogens
• Protective reflexes


  Automatic, involuntary responses to a stimulus that help protect the body from infection
• Protective reflexes


  • Sneezing
  • Coughing
  • Vomiting
  • Diarrhoea
• Sneezing forcefully expels air from the lungs, carrying mucus, foreign particles and irritating gases out through the nose and mouth
• Coughing drives mucus and foreign matter up the trachea towards the throat and mouth
• Vomiting expels the stomach contents, removing irritants
• Diarrhoea removes irritants from the small and large intestines quickly
• Phagocytosis


  The process where specialised white blood cells (phagocytes) engulf and digest micro-organisms and cell debris
• Types of phagocytes


  • Monocytes and macrophages
  • Neutrophils
  • Dendritic cells
• Macrophages are large phagocytic cells that move through tissues looking for and destroying pathogens
• Neutrophils are the most abundant leucocyte and are the first cells to move into tissue to destroy pathogens by phagocytosis
• Dendritic cells detect, engulf and process foreign particles, then use information about the ingested particles to assist with specific immunity