Antigens (AQA A Level Biology)

Antigens

• 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