AQA A Level Biology

Revision Notes

2.5.1 Cell Recognition

Cell Recognition

  • Our bodies have several defence mechanisms against pathogens. These include:
    • Preventing the entry of pathogens by a variety of physical and chemical defences, such as the skin, mucous membranes, tears (containing the enzyme lysozyme, which destroys bacteria) and saliva
    • Inflammation (swelling and heating) of the region invaded by the pathogen, a process known as a non-specific inflammatory response
    • Recognising ‘foreign’ cells and targeting any pathogenic cells, a process known as a specific immune response
  • This ability to recognise ‘foreign’ cells is made possible by specific molecules found on the surface of cells (and viral particles) that enable them to be identified by the body
  • These molecules are usually proteins
    • They are often proteins that are part of the phospholipid bilayer, such as glycoproteins
    • However, glycolipids can also act as similar markers
  • They allow the body to recognise its own cells (‘self’) and foreign cells (‘non-self’)
  • These surface proteins are found on (and enable the body to recognise) the following:
    • Pathogenic cells
    • Abnormal body cells, such as cancerous or pathogen-infected cells
    • Toxins (these are chemical rather than cellular, in nature)
    • Cells from other individuals from the same species (in order for organ transplants to occur successfully the body must not recognise the cells and tissues of the donated organ as foreign, so that no immune response occurs)
  • The surface molecules used by the body to identify cells, viral particles and toxins are known as antigens
  • An example of the importance of antigens in defending against pathogens:
    • White blood cells known as phagocytes have surface proteins that act as receptors and bind to the proteins (antigens) on the surface of pathogens
    • This enables pathogens to be engulfed and digested
    • The antigens that were found on the pathogen can then be presented on the surface of the phagocyte (now an antigen-presenting cell)
    • This is then used to recruit other cells of the immune system, leading to a specific immune response

Exam Tip

Pathogens can cause damage to a host in a number of ways. For example, they can damage host tissue/cells and produce toxins.


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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