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
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
- 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
