Antigens (AQA A Level Biology): Revision Note

Antigens

• All cells, including human cells and microorganisms, have unique markers called antigens

  • Antigens are macromolecules found on cell membranes, bacterial walls, or viral surfaces

  • Glycolipids and glycoproteins on membranes often act as antigens

  • They enable cell-to-cell recognition and help the body distinguish between self and non-self cells

• Antigens can be classified as:

  • Self antigens – produced by the body’s own cells and do not trigger an immune response

  • Non-self antigens – found on foreign cells (e.g. pathogens, incompatible blood cells) and stimulate an immune response

Antigen variability

• Some pathogens (e.g. cold and flu viruses) show antigen variability due to frequent genetic mutations

  • Their surface antigens change, so existing lymphocytes and memory cells can no longer bind due to there being no complementary receptors

  • This prevents a secondary immune response, so the host becomes infected again

• Antigenic variability explains why individuals can catch a cold or flu repeatedly despite past exposure