AQA AS Biology

Revision Notes

2.5.9 Antigen-Antibody Complex

Antigen-Antibody Complex

  • An antigen and its complementary antibody have complementary molecular shapes
    • This means that their molecular structures fit into each other
  • When an antibody collides (randomly) with a foreign cell that possesses non-self antigens with a complementary shape, it binds with one of the antigens
  • When this occurs, the two molecules combine to form an antigen-antibody complex

Antigen-antibody complex

A diagrammatic version of an antigen-antibody complex showing the basic features (in reality, both antigens and antibodies have highly complex 3D structures)

  • As seen in the diagram above, antibodies have at least two antigen-binding sites
  • This means they can bind to more than one bacterium or virus at the same time
  • This cause groups of the same pathogens to become clumped together
    • This process is known as agglutination
  • The binding of antibodies to the antigens either neutralises the pathogen or acts like a marker to attract phagocytes to engulf and destroy the pathogens
    • Due to agglutination, phagocytes can often phagocytose many pathogens at the same time, as they are all clumped together

Agglutinated pathogens cannot move easily,

Agglutinated pathogens cannot move easily, so they are easily engulfed and destroyed by phagocytes

Exam Tip

Remember – antibodies can cause pathogens to stick together (due to their multiple antigen-binding sites), a bit like glue. This might help you remember the biological name for this process: agglutination!


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.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top