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

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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
  • 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, IGCSE & GCSE Biology revision notes



  • 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, IGCSE & GCSE Biology revision notes

How the blood clots

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.