Antigens (AQA A Level Biology)

• Every cell in the human body has markers that identify it
• Microorganisms (both pathogenic and non-pathogenic), such as bacteria and viruses, also have their own unique markers
• These markers are called antigens (which are macromolecules) and they allow cell-to-cell recognition
• Antigens are found on cell surface membranes, bacterial cell walls, or the surfaces of viruses
  • Some glycolipids and glycoproteins on the outer surface of cell surface membranes act as antigens

• Antigens can be either self antigens or non-self antigens:
  • Antigens produced by the organism's own body cells (those that the immune system does not recognise as foreign antigens) are known as self antigens
  • Self antigens do not stimulate an immune response
  • Antigens not produced by the organism’s own body cells (those that the immune system recognises as being foreign eg. the antigens found on pathogenic bacteria and viruses or if a person receives a different blood type during a transfusion) are known as non-self antigens
  • Non-self antigens stimulate an immune response

Antigen Variability

• Some pathogens exhibit antigen variability
  • The antigens present on their surface change frequently due to genetic mutations

• This poses a problem for the immune system of many mammal hosts as lymphocytes and memory cells produce a specific immune response
  • The surface receptors on lymphocytes and memory cells are complementary in shape to only one antigen
  • When the antigen on a pathogen changes the lymphocytes and memory cells can no longer bind
  • As a result, there is no secondary immune response
  • The host gets infected and suffers from the disease again

• The cold virus and flu virus are common pathogens that exhibit antigen variability
  • Individuals can catch the cold and flu year after year as the antigens on the viruses change and are not recognised by their immune system