White Blood Cells & Platelets

• Phagocytes & Lymphocytes:

  Types of white blood cell-

  • White blood cells have a crucial role to play in human defence against disease
  • White blood cells make up less than 1% of total blood volume
  • However, this small number is not a reflection of their importance to our immune system
  • White blood cells are part of the overall immune system, defending against infection by pathogenic microorganisms
  • There are two main types, lymphocytes and phagocytes
• Lymphocytes (around 25% of white blood cells):

  • Lymphocytes produce antibodies to destroy pathogenic cells and antitoxins to neutralise toxins released by pathogens
  • They can also
  • Produce marker molecules to 'mark' pathogens as foreign for phagocytes to recognise
  • Cause pathogens to stick together, making phagocytosis more effective
• Lymphocytes (around 25% of white blood cells):

  • They can easily be recognised under the microscope by their large, round nucleus which takes up nearly the whole cell and their clear, non-granular cytoplasm
  • Lymphocytes have well-developed cell machinery (ribosomes, mitochondria, endoplasmic reticulum) for making new cells, so large numbers of cells can be produced quickly in response to an infection by pathogens
• A lymphocyte:

  
• Phagocytes (around 75% of white blood cells):

  • Phagocytes carry out phagocytosis by engulfing and digesting pathogens
  • Phagocytes have a sensitive cell surface membrane that can detect chemicals given off by pathogenic cells
  • Once they encounter the pathogenic cell, they will engulf it and release digestive enzymes to digest it
  • They can be easily recognised under the microscope by their multi-lobed nucleus and their granular cytoplasm
  • The multi-lobed nucleus allows the phagocyte to change shape easily as it pursues pathogens and carries out the processes of phagocytosis
• Phagocytosis: diagram

  
• Platelets:

  • Platelets are fragments of cells that are involved in blood clotting and forming scabs where the skin has been cut or punctured
  • When the skin is broken (i.e. there is a wound) platelets arrive to stop the bleeding
  • A series of reactions occurs within the blood plasma
  • Platelets release chemicals that cause soluble fibrinogen proteins to convert into insoluble fibrin and form an insoluble mesh across the wound, trapping red blood cells and therefore forming a clot
  • The clot eventually dries and develops into a scab to protect the wound from bacteria entering
• Platelets:

  • Blood clotting is important because:
  • It prevents continued / significant blood loss from wounds
  • Scab formation seals the wound with an insoluble patch that prevents entry of microorganisms that could cause infection
  • The scab stays in place until new skin has grown underneath it, sealing the skin again
• How the blood clots:

  
• How the blood clots:

  
• How the blood clots: