White Blood Cells & Platelets (OCR Gateway GCSE Biology: Combined Science)

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
• 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

• 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
• 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